Stormwater detention, retention and water storage, 1,000 to 30,000 litres. ENDURO is built in the North Island and XPRESS in the South, so your island usually decides the range. Send us the volume off your consent and we will match a tank to it and tell you what the install needs.
What goes over the top, how wet the ground gets, where the outlet has to sit: all of it changes which tank is right. Send finished plans or a rough description of the job. A Promax specialist comes back with a tank and a price, plus the detail your drainlayer will need, at the sale price if you order by 31 October.
There are two underground ranges, and your island usually decides which one. Super Duty and TankGuard are heavier or outlet-protected builds within ENDURO. Pick a range and its sizes, prices and install detail are on that range's own page.
Rectangular, so the dig is a straight-sided trench rather than a round hole. Less excavation, less backfill. Built in the North Island.
Cylindrical, in three diameters. On the 1.2 m the hole stays the same size whatever volume you pick, and only the length changes. Built in the South Island.
The ENDURO built heavier, for a high water table, rocky or heavy fill, or more load over the top.
See Super Duty
Outlet blockage protection for detention, so the tank keeps releasing at the rate the design calls for.
See TankGuardEvery underground tank we make:
Sections are getting smaller and councils now want stormwater managed on site. An underground tank does both jobs at once: it sits under the yard, the driveway or the lawn without touching the building footprint or the landscaping. Promax has been making tanks in New Zealand since 1992, and a rep will spec the job with you before you order.
Standard sizes in both underground ranges are priced 15% below retail on orders placed by 31 October 2026: the twelve 1.0m ENDURO sizes in the North Island, and the nine 1.2m XPRESS sizes plus the 900mm compact in the South. The price on each range page is the sale price, with the retail price alongside so you can see the saving.
Super Duty, TankGuard, the 575mm low profile and the 1.9m XPRESS are priced on enquiry and are not part of the sale.
Every underground tank travels free to your nearest Mainfreight depot, and you can collect from Promax instead if that suits the job. If you want the tank delivered to site, your Promax rep arranges it and quotes it with your order. Tanks of 6,000L and under are delivered free to site within the Auckland Metro area.
Most NZ councils now require new builds and infill developments to slow the stormwater leaving the site, particularly where roofs and driveways cover a large share of it. A detention tank meets that requirement: it sits empty, catches runoff in a storm, and releases it at the rate council allows. Your consent or engineer will state the volume. Send it through and we'll spec the tank.
The volume is not ours to pick. It comes from your resource consent, or from the engineer who did the stormwater design, and it is usually written in litres or cubic metres.
If you have that number, go straight to the North Island ENDURO range or the South Island XPRESS range and pick the size. If you do not have it yet, send us what you do have and we will tell you what is needed to size it.
Geography and shape, not budget. ENDURO is rectangular, built in the North Island, and runs 1,000 to 10,000 litres. XPRESS is cylindrical, built in the South Island, and runs 1,000 to 30,000 litres. Both cover detention, retention and storage.
The 1.2m and 900mm XPRESS are sold in the South Island. The 1.9m XPRESS is sold nationwide, with freight quoted with your order. For a North Island site, start with ENDURO.
A detention tank holds stormwater during a storm and releases it gradually through a controlled outlet. It slows the rate of discharge from your site, which is commonly required by council.
A retention tank stores rainwater or stormwater for later reuse: irrigation, toilet flushing or washdown, depending on the system design. The tank is the same one. What changes is the outlet arrangement and the fittings.
A dual-purpose tank does both in one unit. A 4,000 litre tank, for instance, can hold 1,850 litres of detention in the upper section and 2,150 litres of retention below it, so the council requirement and the water you reuse come out of a single install.
Only with the right lid and riser. The standard polyethylene lid supplied with the tank is for garden areas. For a pedestrian or trafficable area the installation guide calls for a steel manhole lid and a Promax adjustable riser, which takes a concrete surround so vehicle loads are carried by the concrete and not the tank.
Tell us what goes over the top, whether that is foot traffic, cars or heavy vehicles, and we will confirm what the install needs before you dig.
An empty tank in saturated ground wants to lift. Uplift restraint kits and installation anchors hold it down, and they are sized to the tank: one kit per 1,000 litres on the 1.0 m ENDURO and the 1.2 m XPRESS, so a 5,000 litre tank takes five, and one per 2,500 litres on the 1.9 m.
Stated burial depths in the installation guide assume dry ground with no hydraulic loads, so if groundwater is a known issue on your site, say so when you enquire and it goes into the spec.
When the site has a high water table, rocky or heavy soil, flotation risk, or more load over the top than a standard install. It runs 2,000 to 10,000 litres, it is part of the North Island ENDURO range, and it is usually specified alongside an uplift restraint kit. Send your site details and we'll confirm whether it applies.
TankGuard suits stormwater detention projects where protecting the controlled discharge outlet from blockage is part of the design. It is built as a separate version of the tank rather than fitted afterwards, so ask us and we will quote the TankGuard version of the size you need.
Pro-rata, from the date of purchase, per the Promax Warranty Terms and Conditions:
The warranty covers the tank. Lids, fittings and inlet and outlet connections are excluded. The tank must be registered on promax.co.nz within 30 days of delivery.
A drainlayer, working to the Promax Underground Tank Installation Guide that comes with the tank. Excavation, bedding and backfill all matter on an underground tank, and connecting into the drainage is restricted work in New Zealand, so it has to be done by a registered drainlayer.
Your project location, the application, the required volume, site conditions, consent notes, and any plans or outlet specifications you have. Nothing needs to be finalised before you make contact.
Still got questions? Talk to the team →