Stormwater Detention Tanks

Promax stormwater detention tanks are designed to temporarily hold stormwater during a rain event and release it slowly to the stormwater network at a controlled rate. They are commonly used to help meet council stormwater requirements on residential and commercial developments across New Zealand. Promax offers both underground detention tanks and slimline detention tanks, giving you practical options for sites where space is limited, above-ground storage needs to stay discreet, or the required detention volume is relatively small.

Underground tanks are often the best option where the tank needs to sit below a lawn, driveway, or parking area. Slimline tanks are a strong alternative for tight residential sites where the required volume is modest and there is space along a fence line or beside the house. Manufactured in New Zealand, Promax stormwater tank solutions are designed to help builders, plumbers, drainlayers, developers, and homeowners manage runoff in a practical and site-efficient way.

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1,000L Underground Tank 575mm Dia - ENDURO

P.O.A

1,500L Underground Tank 575mm Dia - ENDURO

P.O.A

2,000L Underground Tank 1m Dia - ENDURO

$2,539.00

2,500L Underground Tank 1m Dia - ENDURO

$2,933.00

3,000L Underground Tank 1m Dia - ENDURO

$3,327.00

3,500L Underground Tank 1m Dia - ENDURO

$3,595.00

4,000L Underground Tank 1m Dia - ENDURO

$3,863.00

4,500L Underground Tank 1m Dia - ENDURO

$4,203.00

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Why You Need a Stormwater Detention Tank

As more New Zealand sections are covered by roofs, paving, driveways, and hard landscaping, less rainfall soaks naturally into the ground. Instead, water runs off quickly and enters the public stormwater system. During heavy rain, this can overload downstream infrastructure and increase the risk of local flooding.

That is why many councils require new developments to manage stormwater on site. A stormwater detention tank helps by storing runoff during the rain event and releasing it slowly afterwards through a controlled outlet. The goal is to reduce peak discharge and ease pressure on public stormwater networks.

On many sites, especially new urban builds, the challenge is not just compliance but space. Some sections have no practical room for a large round tank. Others only need a relatively small detention volume. That is where Promax gives you two effective options: underground stormwater detention tanks for concealed below-ground installation, and slimline detention tanks for narrow above-ground locations where a compact footprint matters.

What Is a Stormwater Detention Tank?

A stormwater detention tank is designed to live most of its life empty. During a rain event, water flows into the tank faster than it can leave through the outlet, so the tank fills temporarily. It then drains down gradually over time.

This slows the release of stormwater into the public network and reduces sudden surges of runoff. In simple terms, a detention tank helps protect the stormwater system by spreading discharge over a longer period.

That is different from a stormwater retention tank, which is designed to hold water for reuse.

Detention vs Retention: What Is the Difference?

These terms are often confused, but they describe different functions.

A stormwater detention tank:

Temporarily stores runoff during rain
Slowly releases it to the stormwater network
Is generally empty again after the event

A stormwater retention tank:

Stores water for later use
Keeps a standing water volume
Can be used for irrigation, toilet flushing, washdown, and other reuse applications depending on the system design

In short, a detention tank reduces pressure on the stormwater system, while a retention tank reduces pressure on the water supply system.

When a Stormwater Detention Tank Is the Right Solution

A detention tank is often the right choice when:

Council requires on-site stormwater detention
The site is a new build, infill development, or redevelopment
Hard surfaces have increased stormwater runoff
The site has limited room for conventional stormwater storage
The required detention volume is small to moderate
The tank needs to be hidden underground or kept unobtrusive above ground

Underground Detention Tanks

Promax underground tanks are a strong solution for detention projects where above-ground installation is not practical or where the site layout needs to remain clean and usable. Installed below a lawn, driveway, or parking area, they allow the development to meet stormwater requirements without sacrificing outdoor space.

Promax underground detention tanks are available in:

ENDURO Underground Tanks

1,000L to 10,000L
10-year pro-rata warranty
Rectangular form for efficient underground storage
Available with Super Duty and TankGuard options for more demanding conditions
Manufactured in Kerikeri for the North Island

XPRESS Underground Tanks

2,000L to 30,000L
5-year pro-rata warranty
Larger capacity options for bigger residential and commercial projects
Manufactured in Rangiora for the South Island

Underground detention tanks are especially useful when:

There is no room beside the house or boundary
The tank must sit under usable site space
The detention volume required is too large for a compact above-ground option
The project requires a concealed stormwater solution

Slimline Detention Tanks

Slimline tanks are also a practical option for stormwater detention, especially on compact residential sites where the required detention volume is small and there is limited room for a traditional round tank.

Promax Slimline Tanks are designed for tight New Zealand urban spaces. Their long, narrow shape allows them to fit beside a house, along a fence, or in other narrow areas where a standard tank would be too bulky. This makes them particularly relevant on sites under 500 m², where builders and drainlayers often need a detention solution that works around setbacks, accessways, and boundary constraints.

Promax slimline tanks are available in:

1,000L
2,000L
3,000L
5,000L

For detention applications, slimline tanks are best suited where:

The required detention volume is relatively small
The site does not justify underground excavation
There is usable wall or fence line space
A low-profile and visually discreet installation is preferred

Slimline tanks also offer practical installation advantages. They include integrated seismic restraints and are designed to bolt to a concrete pad, which can simplify installation compared with some alternative tank systems. Their low-profile shape also helps where height-to-boundary constraints matter.