Septic tanks - black, grey, storm water

Septic tanks - black water, grey water & storm water

Image: Wright Tanks

In this workshop we brought together experts and local knowledge in the area of septic tanks and stormwater, held at St. Peter’s Hall, Paekākāriki, Sunday 31 May 2026.

What we learnt about septic tanks and the drip lines and other systems that dispose of the wastewater is that your needs are extremely dependable on both what and where you plan to build, and the land you have: space, soil and slope.

Incinerator and compost toilets provide other options and some notes from discussion and some links are provided at the end of this webpage.

Our panel...

Greg Cundy | Kiwi Green Plumbing | www.septictanks-kapitihorowhenua.co.nz
Robert Wright | Wright Tanks | www.wrighttanks.co.nz
Ben Thompson | Kāpiti Coast District Council Water Conservation and Trade Waste Officer | www.kapiticoast.govt.nz

Note the following does not represent professional advice - please contact the above businesses directly for that.

We recommend you also read this basic government guide.

Septic tanks and stormwater

Old septic tank systems in Paekākāriki vary from old brick ones that are really subpar, to concrete ones that have been working well for many decades.

We understand our old systems are ‘passive systems’, often one chamber, where water then goes out to stone fields after effluent settles.

Passive systems are still possible to have installed, but these are not as good for the environment as multichambered systems with electrical pumps which get called AWTS (aerated wastewater treatment systems).

These use oxygen to treat household sewage into clean water for garden use, spread out over an area of garden via driplines.

Kiwi Green Plumbing and Wright Tanks have installed new systems in Paekākāriki in the past, and have decades of experience on the Kāpiti Coast.

Many residents also get their systems assessed by JB’s.

Greg Cundy from Kiwi Green notes that Paekākāriki’s particular challenge with septic tanks compared to properties not on the council sewerage system further north is space - we don’t have bigger rural sections.

Kiwi Green note that...

Septic tanks need to be a minimum 3 metres from the house, 1.5 metres off the boundary, and need to have a 1:60 fall or drop from the house
A drop line irrigation effluent field (more explanation below) has to be 5 metres off the boundary or 1.5 metres with a resource consent
The field needs to be 20 metres from any waterways, creeks, drains or bores
These drip lines are buried 100 to 150 mm down or under mulched and planted raised beds where there are water table or ground water issues.

These conditions are set by Greater Wellington Regional Council “then separate councils have their separate opinions.”

Kiwigreen do a lot of servicing of existing tanks, and the tanks they install they buy off the shelf and have expertise installing them.

Wright Tanks design and engineer a system for you from scratch which they then service, or can do retrofits of existing tanks which is another option

(see discussion below).

Robert Wright, Wright Tanks: “We're here to help and improve the environment. Because if you know the coastal area, there’s a high water table which requires local experience because it creates risks for septic systems.”

Ben Thompson, Water Conservation and Trade Waste Officer at KCDC, also notes up front that - since a plan change in 2008 - you will also need a rainwater tank for toilets on the Kāpiti Coast whenever you build a new dwelling or relocate a dwelling to a new site.

Which are the different features of a septic tank system? How much area do you generally need? And how do they vary?

Greg, Kiwigreen: If it’s a brand new septic tank, the size depends on the occupancy, and the size of the home. Septic tank installers have a whole lot of regulations and guidelines that we have to stick to. If you are putting in another dwelling, and you are going to connect into your septic tank you will need a council building consent.

The engineers and designers come up with the actual size. The regulations state how much the minimum should be.

The needs vary depending on the property so advice on size here is purely a ballpark.

A 3-4 bedroom house has traditionally required 3,000 litres. But once you go into AWTS - secondary treatment systems - it can be between 7,000 - 10,000 litres.

Generally, the smallest unit that Kiwigreen are dealing with at the moment for an aerated, secondary treated system is 7,000 litres.

So these fit all bedroom sizes (or at least up to 5-6 bedrooms). They’re all variable sizes depending on the depth and water table.

Editor note: While an AWTS actively pumps air into a tank to feed bacteria, the traditional passive systems rely on natural gravity and airflow through rocks.

Greg: You can also have pumps in passive systems - most of the modern tanks have pumps. A lot of people don't want to use electrical pumps as they wear out and need service. But it’s about quality. You’re trying to get the cleanest effluent back in the ground. And if you’re talking about a small property that doesn’t have a lot of space, these things become more difficult.

The majority of sites they’ve been to in Paekākāriki have old concrete septic tanks about 3,000 litres in capacity.

There’s generally also not a lot of space - for a modern tank a three-bedroom house needs 300 square metres for the dripline. That’s not always possible so you've got to put a smaller field in and somebody has to design that for your particular property.

So, how do AWTS’s work?

Greg: If you flush the toilet, six litres of water goes into your primary chamber, with the effluent sitting in the middle and allowed to settle. The water will then jump into the next chamber; generally: an aeration chamber. In there it's like a spa pool, bubbling up from the bottom. The aerator is running all the time, but in power it's the same as running an 80 watt light bulb. From that, about 6 litres of water then goes into a clarification chamber. That's where it is calmed and settles down and, if there's any particles left they settle - and then six litres moves over into a pump chamber.

That pump chamber builds up to about 400 litres of water, and when it hits that, the pump kicks in, pumps it out, and that goes out into the effluent field. That field could be a drip line, it could be a stone bed, or whatever has been designed for you.

How long does that process take?

Greg: It's all done by the volume that you're using within your house. A rough calculation would be every person using 200 litres of water per day (including toilets, showers, washing machines and that which is wasted). A modern system fills up to about 400 litres of water and then kicks it out for 10 minutes, filling the field and then turning off. So it's very quick and efficient. That's what you're paying for. You're paying for that engineered design that's going to work every single time.

Robert Wright, Wright Tanks: The size of system goes on the number of people - they work out the amount of solids that go through a system based on this. Our treatment plant will comfortably do 2,000 litres of effluent a day. And we've got a smaller system that we use for a two-bedroom dwelling.

“When we design it we work out how many occupants are on the property. If you are on town supply water, then the design rate goes up because it means you’ve got endless water - so we've got to design on a higher loading rate. Then that will determine how many metres of drip line we need to keep it under a 3 litre per square metre per day loading rate (Note: this is the Greater Wellington Regional Council requirement)."

“The loading rate comes down to different soils, which can also affect the type of pump - which might need to be a slower emitter. Even some soils, you can get down to like a 2.6 litre loading rate, and during the winter, it can still be very boggy. That's just because of what happens under the ground - you might, for example, have a hill behind you."

“Over the years, you get to know the areas where you're working, what will work. Sometimes I'll get down to just a 1.8 litre loading rate, because during the winter it will give you endless amounts of trouble. Which is why the councils sometimes stipulate that you have to plant the fields up. But when you get a 600 square metre disposal field that's a lot to plant up. And sometimes you haven't got 600 square metres surplus."

“Then there can be issues with the groundwater table. If you haven't got the room for a drip line system we have to put in a trench system. Normally your trenches will be around 400mm deep, and from that 400 you've got to drill down another 1200. If you strike water, then that's it. Then you've got to redesign it again because they won't accept it.”

Image: Wright Tanks septic system being installed in Paekākāriki

Will adding an additional dwelling sometimes mean additions to the system because of its distance

from the septic tank or a slope?

Robert: “Sometimes you need to add a maceration chamber (which reduces solids to small pieces) and you've got to pump it to the system."

“So when you're looking at doing a project with a granny flat, it pays to get some advice. It doesn't actually cost anything for us to come out and do a report. We can give you some advice. And if it gets to the point where we can’t help you - then it's pretty much the point where you won’t be able to do it.”

“The company that I work for, it's my brother's company. At the end of the day, it's our name. And it's the same with Kiwi Green. You've got to stand by what you put in the ground."

“There's a lot of companies out there, they will buy a tank off a manufacturer, like a plumber and once it's in the ground, and they've been paid, they walk away. There's a lot of people out there that are only interested in putting them in the ground. But it's like anything - it's like a car. If you don’t service anything that you have, it will break down."

“Effluent is the same. Effluent is unforgiving. It really is. After the amount of time that we've spent in the trade, we know what can go wrong.”

Greywater Systems

Greywater is wastewater from household sources like showers, baths, bathroom sinks, and washing machines. It does not contain human waste or raw sewage (this and kitchen sink material is called blackwater).

Greg: Most septic tanks can also take grey water but Greg is “not a great fan".

Grey water's got a lot of pathogens in it and I believe there’s a lot of rubbish in them from what’s tipped down the sink. When you have to clean or service those they're pretty dirty.”

Robert: “Greg's absolutely right. It's convenient. But if you've got a standard system tank with a disposal field and then you want to put a grey water system in, then you've got to have another system for your grey water with a drip line. So then it gets to the point where if you've already got no space, where are you going to put the discharge for the grey water?”

Editor note: some people have managed to put greywater systems in in Paekākāriki so it’s worth exploring whether you have the space to do so.

Ben Thompson, have you got some thoughts on that?

Ben: “Paekākāriki is quite unique because you have to deal with your blackwater (effluent) as well. Most of the systems where the grey water systems have gone in have been where they are connected into a sewer plant."

“The reason we brought grey water as an irrigation system in was to offset town water demand. It wasn't a way of making your septic system perform better.

But if you can get your shower, bath and washing machine - and not your sink which has got your cleaning products - out of that current septic system, you are going to slow the movement down and give it more chance for the solids to settle and improve its performance.

By taking those sources of water and targeting outdoor areas you can potentially get that better use. But in Paekākāriki there is a challenge in doing both.

There is a paper that ESR did, where Dale Wills trialed a water smart system under his natives and fruit trees and managed to get that in with an existing septic system, but he wasn't building an extra home.

Retrofitting...many of us have old septic tanks - how many can be kept going, working with what we already have, or improving things like our drainage fields rather than buying new systems?

Robert: “A new system is easy, because you dig a hole, you put in a tank, and you start a new field. Then you've got to suck the old one out, and then you've got power, and you've got drainage."

“Normally when someone builds a house, they want their new kitchen - the first thing that comes to mind is that they need water. So they go out and get some water tanks. And they think about the landscaping and the driveway and everything. The waste water is normally the last thing they actually think about. Because who wants to think about waste water?! But the trouble is, for the cost of putting a treatment plant in, and what it actually physically does, it keeps the household going, basically. So what I'm trying to say is: easy, put in a new system.”

“But if you don't have the budget or don’t have the room, we can do a retrofit. So the first thing is we have to suck the tank out. You've got to inspect it and see if the tank's watertight and the soil isn’t contaminated. With a single chamber system we cut a hole in the roof and fit a filter, and put a pump station in after that it can go to a new disposal field. Just because the system is backed up or not working doesn't mean you have to replace the whole thing. Before you put your hand in your pocket I need to make sure you don’t have any other option. Because I know, times are tough."

“But you've got to also take into account that if you've got a passive feed system, basically, once you get over 600 millimeters in depth you've got to go through a pump system anyway. And if it's a new home, you automatically, it's a new system."

"But the main thing: get some advice, it can save you thousands.”

Maintenance

Kiwi Green recommendation on their website: that your system gets professional tank services every 6 months to ensure it is working effectively, your warranty is kept valid and that your wastewater is of an acceptable health standard.

Robert: "For a ‘suck out’ its best every 3-5 years, but you need to service your pump station as it collects sediment and that will lead to your dripline stop working."

Boundaries

Space is also an issue due to boundary restrictions.

Greg: “You have to meet GWRC requirements with your drip lines being five metres off your boundary. Which may just leave this little strip in the middle where you’re going to build your house. And then don't forget your three metres setback from the house as well."

“So unless you’re going to put it on wheels - there's a little trick, because now it's a caravan, it's portable - we still have those rules. We have to try and tick all these regulation boxes. We're not going to put it on your boundary because you're not allowed, and we'll be the first ones to get the phone call. It's not worth us as a business, as an industry, to do that."

“And that's just the irrigation field. So then you've got your septic and if you're building new as Ben will tell you, you'll need a 10,000 litre water tank on a new building."

Water tank requirement

Ben: "An existing sleepout converted into a separate dwelling will not necessarily require a separate water tank."

Under the amendments to the ‘granny flat legislation’, the exemptions only apply to new builds.

If the building was lawfully established, did not require a water tank when it was built, and its roof area or footprint is not being increased, a tank may not be required.

However, the conversion will require building consent, and stormwater must be managed so the amount and rate of runoff from the site do not increase.

A stormwater specialist will need to assess the site and recommend an appropriate solution. Depending on factors such as soil soakage, this could require a tank and soak pit, a soak pit alone, or use of an existing council-approved kerb outlet where no additional runoff is being directed to it.

This differs from a new dwelling built under the 70 m² granny flat exemption, which requires a 10,000-litre water tank unless resource consent is obtained for a smaller tank, supported by suitable calculation (can meet the realistic demands of flushing the toilets and outdoor demand).

In some ways our district plan hasn't kept up with intensification, so there is flexibility there where you just need to show that you can meet a 30% water saving on your property.

If you present an argument to say, hey, look, my dwelling, can't collect 10,000 litres. I've only got a 50 square metre roof. The house is too small for the tank. And also these are the measures that I'm doing to save the 30% of peak use. We've developed a bit of a table to provide a guide, because some of the section sizes are 70 square metres that are coming through. People need a home and the market is responding.

Stormwater control

Ben: “Pretty much however you dealt with stormwater before you built that secondary dwelling, is how we'd like you to see that stormwater coming off your property. The easiest way is a soak pit."

"You'll work through that normal process with your plumber, your drainlayer, to calculate a soak pit that can handle a 100 year storm. But if you live at the bottom of a sand dune and your water table's too high, probably what you're going to be looking at is an attenuation tank."

"What an attenuation tank does is it holds the water inside and slowly releases it. It’ll surge up in that tank and slowly release out.”

Paekākariki space issues

Robert: “I'm all for the granny flats. I think it's a great idea. But the trouble is when you've got Paekākāriki and other coastal areas (off the district’s sewerage system) you've got nowhere to discharge. Ideally what you want to do is you want to keep all your solids within your property and you pump all your grey water out. But you haven't got the area and you haven't got the infrastructure here for it. And Paekākāriki is growing. All the coastal areas are growing."

Ben: “We did do a study a while back. We got a consultancy firm in. We did a price to see how much it would cost to do a reticulated sewerage system for Paekākāriki. It's not cheap. It's in the tens of millions. That would be a big conversation to have.”

Editorial note: Ben Thompson later shared the estimates from this 2022 KCDC high level study to estimate what sewerage reticulation connecting Paekākāriki to the Paraparaumu Wastewater Treatment Plant would cost. The estimate at that time was between $66 and $84 million dollars.

“For them to do that, the council would have to be absolutely on board to be piping it all the way to Raumati. And then obviously the infrastructure within Raumati to get it to the plant.”

Ben also notes that KCDC are in the process of reconsenting their plant for Paraparaumu and Waikanae with some asset renewal over the next 10 to 15 years, which will be a focus. And there is also discussion about putting a second line in between growing Waikanae and Paraparaumu.

Can putting an additional dwelling in trigger the need to entirely get your septic tank system

up to a modern standard?

This is likely if you don’t have the capacity. The other option would be to put in a separate system for the additional dwelling - but that would depend how much land you’ve got.

Robert also thinks it might not be long before they bring in Warrants of Fitness for septic tanks. It’s being pushed for in Auckland: “It’s coming.”

After this session we moved to...

A Paekākāriki community Response Panel

Johnny Wraight on compost toilets and the use of biochar, where Wraight questioned why we add water to our waste in the first place and discussed other options
Anne Woodside on a brand new additional dwelling
Kim and Aimee Crothall on the start of their additional dwelling journey.

Incinerator and Composting Toilets

In discussion time there was useful conversation about incinerator and compost toilets.

A resident at the workshop has an incinerator toilet that cost $5000 and was council approved.

We’ve found a guide to incinerator toilets and other models (as one example) from a commercial provider online HERE.

Johnny Wraight mentioned that whole septic tank systems might cost you $20,000 whereas an off-the-shelf composting toilet can cost you

$2000-$3000 (there are many providers but here is one range example), or a two bucket system for very little (see his longer talk here).

There are also wooden outhouse style compost toilets you can buy for an initial $460 (see this example).

GWRC have a useful initial compost toilet guide HERE.

Waste must be able to be disposed of on the property and can’t be used for 12 months. There are other conditions and approval is needed from council.

This can be tricky and the housing trust needs to be cautious about giving advice.

It is hard to get one straight answer from council because it is site specific and design specific, as septic tanks are. This is work the housing trust are interested in pursuing clearer guidelines on.

Johnny: “In Paekākāriki our culture is the sepo. And if you can keep your old one going, that’s good. But on a note, with septic tanks, most people don't realise that anaerobic digesters and a by-product of that is methane, a hell of a lot of methane, and it's way, way more damaging as a global climate change gas than CO2. In the drain fields you get nitrous oxide emitting, which is way worse than CO2 as a climate change gas. So with the Sepo system, all us greenies in Paekakariki, we're probably one of the biggest emitters of greenhouse gases in the district. Just saying…“

“I would love to see just a simple dry system. You still have to deal with your grey water, but you can do that with these soakaways, you can get a grey water system. But if you just take the shit out of the mix, dry it out and deal with it, it's a resource. And then you don't have to have a septic tank. You can build where the sepo was. You've got all this land. These drain fields just rob us of land.”