Underground Tanks Underground Tanks

Underground Tanks

Promax underground tanks store water below ground where above-ground installation is not possible or permitted. Council requirements, space constraints, stormwater management: all resolved with a single solution. Available in the ENDURO and XPRESS series from 1,000L to 30,000L, with a full range of risers, lids, anchor systems, and installation kits. Made in New Zealand at our facilities in Kerikeri and Rangiora.

Promax underground tanks are used for stormwater detention and retention, potable water storage, and fire water supply. A single tank can serve two purposes: detaining excess stormwater during rainfall and storing water for household reuse.

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6,000L Underground Tank 1.2m Dia - XPRESS

6,000L Underground Tank 1.2m Dia - XPRESS

$5,169.00
$5,169.00
7,000L Underground Tank 1.2m Dia - XPRESS

7,000L Underground Tank 1.2m Dia - XPRESS

$5,622.00
$5,622.00
8,000L Underground Tank 1.2m Dia - XPRESS

8,000L Underground Tank 1.2m Dia - XPRESS

$6,226.00
$6,226.00
9,000L Underground Tank 1.2m Dia - XPRESS

9,000L Underground Tank 1.2m Dia - XPRESS

$6,685.00
$6,685.00
10,000L Underground Tank 1.2m Dia - XPRESS

10,000L Underground Tank 1.2m Dia - XPRESS

$7,134.00
$7,134.00
10,000L Underground Tank 1.9m Dia - XPRESS

10,000L Underground Tank 1.9m Dia - XPRESS

P.O.A
P.O.A
15,000L Underground Tank 1.9m Dia - XPRESS

15,000L Underground Tank 1.9m Dia - XPRESS

P.O.A
P.O.A
25,000L Underground Tank 1.9m Dia - XPRESS

25,000L Underground Tank 1.9m Dia - XPRESS

P.O.A
P.O.A
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Why You Need an Underground Tank in NZ

Section sizes in New Zealand cities are shrinking. In Auckland, the average section size has dropped by a third over the past three decades. Sections under 500 m2 are now the norm, and some sites have no room at all for an above-ground tank. An underground tank solves this: it sits beneath the yard, driveway, or parking area and takes up zero space above ground.

The second factor: mandatory council stormwater management requirements. In most New Zealand cities, new developments must manage stormwater on site. The reason is straightforward: hard surfaces (concrete, asphalt, roofing) do not absorb rain. Water runs off into the stormwater network and overloads it during heavy rain. Councils address this by requiring developers to install tanks for stormwater detention or retention.

Promax underground water tank installed on site for stormwater storage.

What an underground tank gives you

An underground tank frees up the entire site. It works beneath the yard, driveway, or lawn without affecting the building footprint or landscaping. For sites under 500 m2, this is often the only way to meet council requirements while keeping space for the house and garden.

A dual-purpose tank solves two problems at once: it meets the council's mandatory stormwater detention requirement and stores water for domestic reuse. One investment instead of two separate systems.

An underground tank requires no maintenance over its service life. Polyethylene does not rust, does not crack underground, and does not need recoating. The warranty of up to 10 years (ENDURO) confirms that the manufacturer stands behind the durability of the product.

Promax underground water tank installed during the construction.

Detention tank vs retention tank: what is the difference

These two terms are often confused, even though they describe fundamentally different functions.

A detention tank lives empty. During a rain event it collects stormwater and slowly releases it into the municipal stormwater network at a controlled rate. The goal: reduce peak loading on municipal infrastructure and prevent flooding. After the rain, the tank is empty again and waiting for the next event.

A retention tank stores water for reuse: for laundry, toilet flushing, and irrigation. It reduces consumption from the municipal water supply.

Promax offers dual-purpose tanks that perform both functions in a single unit. For example, a 4,000L tank can have 1,850L of detention volume in the upper section and 2,150L of retention volume in the lower section. This way the developer meets council requirements and at the same time gives the homeowner a practical benefit from the collected water.

Groundwater: hydrostatic pressure and flotation

Groundwater is the most underestimated risk when installing an underground tank. Two separate issues:

Hydrostatic crush pressure: groundwater pushes against the walls of an empty tank from the outside and can crush it. Solution: the ENDURO Super Duty tank is certified to AS/NZS 4766:2020 for operation under permanent external pressure, or install drainage around the tank.

Flotation: groundwater pushes an empty tank out of the ground. Solution: an Uplift Restraint Kit. For tanks up to 10,000L with a 1 m diameter: one kit per 1,000L (a 5,000L tank requires five kits). For 1.9 m diameter tanks: a separate Uplift Restraint Kit for 1.9 m diameter.

Where there is a groundwater risk on site, Promax assumes by default that groundwater may rise to surface level. Promax can supply PS1 documentation for seismic and underground projects (available at additional cost, contact us for details).
Promax underground tank being installed for water storage. Promax underground tank being installed for water storage.

ENDURO series: 1,000L to 10,000L, 10-year warranty

The ENDURO underground range covers volumes from 1,000L to 10,000L with a 10-year pro-rata warranty. Manufactured at the Kerikeri factory and available on the North Island.

Diameters:

  • 1,000L and 1,500L: 575 mm diameter
  • 2,000L to 10,000L: 1 m diameter

The rectangular shape of ENDURO gives an advantage over cylindrical tanks: more storage volume in a smaller perimeter, shallower depth, less excavation. In a project in Epsom, Auckland, two 10,000L ENDURO tanks replaced a cylindrical configuration, saving 13 to 15 m3 on excavation and 15 to 20 m3 on backfill.

Reinforcement options (2,000L to 10,000L)

  • Super Duty: reinforced construction for increased loading. The only underground tank in New Zealand certified to AS/NZS 4766:2020 for operation under permanent hydrostatic pressure.
  • TankGuard: protective barrier for challenging ground conditions.
  • Super Duty TankGuard: combines both features for the most demanding conditions.

All ENDURO variants (including Super Duty and TankGuard) carry the same 10-year pro-rata warranty.

XPRESS series: 2,000L to 30,000L, 5-year warranty

The XPRESS underground range covers larger volumes and larger diameters. Manufactured at the Rangiora factory and available on the South Island.
Volumes and diameters:

  • 2,000L to 9,000L: 1.2 m diameter
  • 10,000L: available in 1.2 m and 1.9 m (choice depends on site conditions)
  • 10,000L to 30,000L: 1.9 m diameter

At 30,000L, the XPRESS series is the largest single underground tank in the Promax range. XPRESS underground tanks carry a 5-year pro-rata warranty.
A standard 2,500L underground tank (1.9 m diameter) is also available outside the ENDURO and XPRESS series.