Stormwater Retention Tanks Stormwater Retention Tanks

Stormwater Retention Tanks

Promax stormwater retention tanks are designed to collect and store water for later use rather than releasing it straight into the stormwater system. They are used on residential and commercial sites where captured rainwater or stormwater can be reused for irrigation, toilet flushing, washdown, and other non-potable purposes depending on the system design. Promax offers both underground retention tanks and slimline retention tanks, giving you flexible options for sites where space is limited, visual impact needs to be reduced, or a modest storage volume is all that is required.

Underground tanks are well suited to concealed storage below lawns, driveways, and parking areas. Slimline tanks are ideal where space is tight but there is still room beside the home or along a boundary. Manufactured in New Zealand, Promax retention tank solutions help homeowners, builders, plumbers, and developers make better use of collected water while reducing reliance on mains supply.

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9,000L Underground Tank 1m Dia Super Duty TankGuard - ENDURO

9,000L Underground Tank 1m Dia Super Duty TankGuard - ENDURO

P.O.A
P.O.A
9,000L Underground Tank 1m Dia Super Duty - ENDURO

9,000L Underground Tank 1m Dia Super Duty - ENDURO

$9,497.00
$9,497.00
8,000L Underground Tank 1m Dia TankGuard - ENDURO

8,000L Underground Tank 1m Dia TankGuard - ENDURO

P.O.A
P.O.A
8,000L Underground Tank 1m Dia Super Duty TankGuard - ENDURO

8,000L Underground Tank 1m Dia Super Duty TankGuard - ENDURO

P.O.A
P.O.A
8,000L Underground Tank 1m Dia Super Duty - ENDURO

8,000L Underground Tank 1m Dia Super Duty - ENDURO

$8,845.00
$8,845.00
7,000L Underground Tank 1m Dia TankGuard - ENDURO

7,000L Underground Tank 1m Dia TankGuard - ENDURO

P.O.A
P.O.A
7,000L Underground Tank 1m Dia Super Duty TankGuard - ENDURO

7,000L Underground Tank 1m Dia Super Duty TankGuard - ENDURO

P.O.A
P.O.A
7,000L Underground Tank 1m Dia Super Duty - ENDURO

7,000L Underground Tank 1m Dia Super Duty - ENDURO

$7,986.00
$7,986.00
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Why You Need a Stormwater Retention Tank

A stormwater retention tank turns rainfall into a usable resource. Instead of sending captured water straight into the public stormwater network, the tank stores it so it can be reused around the property.

That can make a real difference on New Zealand sites where outdoor water use, irrigation, washdown, or household reuse creates ongoing demand. For many property owners, a retention tank is a practical way to reduce dependence on mains water while improving the value of the water already falling on the site.

As section sizes shrink and building footprints grow, storage space becomes more limited. Some sites have no room for a large round tank. Others only need a relatively modest amount of retained water. Promax addresses this with both underground retention tanks and slimline retention tanks, so the right storage format can be matched to the site.

What Is a Stormwater Retention Tank?

A stormwater retention tank stores water for later use. Unlike a detention tank, it is not meant to drain empty after the rain event. Instead, it keeps a standing reserve of stored water that can be drawn down when needed.

This retained water may be used for:

  • Irrigation
  • Garden watering
  • Driveway washdown
  • Vehicle washing
  • Toilet flushing
  • Other reuse applications depending on the system design

A retention tank is designed to serve the property owner directly. It gives you practical access to stored water rather than simply controlling discharge to the network.

Retention vs Detention: What Is the Difference?

A retention tank:

  • Stores water for reuse
  • Keeps a standing reserve
  • Reduces demand on mains supply

A detention tank:

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

Both are stormwater tanks, but they are set up for different purposes. A detention tank helps protect the public stormwater system. A retention tank helps provide usable water for the property.

When a Stormwater Retention Tank Is the Right Solution

A retention tank is often the right choice when:

  • The project wants to reuse stored stormwater or rainwater
  • The property has ongoing water demand for irrigation or washdown
  • The owner wants to reduce reliance on mains supply
  • Above-ground space is tight but some storage is still needed
  • The tank must be hidden below ground or kept visually discreet
  • The project is considering a combined detention and retention setup

Underground Retention Tanks

Promax underground tanks are a strong retention solution where storage needs to be concealed or where above-ground space is unavailable.

Installed below a lawn, driveway, or parking area, they allow the site to retain useful water without affecting the landscape or building footprint above ground. This makes underground retention tanks especially useful on compact urban sites, higher-end residential projects, and developments where visual impact matters.

Promax underground retention tanks are available in:

ENDURO Underground Tanks

  • 1,000L to 10,000L
  • 10-year pro-rata warranty
  • Rectangular shape for efficient underground storage
  • Optional Super Duty and TankGuard variants for more demanding conditions
  • Made in Kerikeri for the North Island

XPRESS Underground Tanks

  • 2,000L to 30,000L
  • 5-year pro-rata warranty
  • Larger capacities for bigger retention requirements
  • Made in Rangiora for the South Island

Underground retention tanks are typically the better option when:

  • Above-ground tank placement is not possible
  • The required retention volume is larger
  • The owner wants the tank fully concealed
  • The project already involves excavation or underground services

Slimline Retention Tanks

Slimline tanks are also an excellent option for stormwater retention, particularly on smaller residential properties where the storage requirement is modest and space is limited.

Promax Slimline Tanks are designed for compact New Zealand sections and can be installed beside the house or along a boundary with minimal visual impact. Their narrow footprint makes them ideal where a standard round tank would take up too much room.

Available capacities include:

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

This makes slimline tanks well suited to:

  • Urban new builds
  • Townhouses and smaller residential lots
  • Side-yard installations
  • Homes that want practical water reuse without a large tank dominating the site

For many homeowners, a slimline retention tank is one of the simplest ways to collect and use rainwater on a tight section.