Plastic Detention Tanks

How detention tanks work

A detention tank temporarily stores roof or surface run-off and releases it slowly back to council infrastructure. By shaving the peak off heavy rain events, the system reduces flooding risk, pipe surcharge and scouring of natural waterways. Unlike retention tanks, the detained water is not intended for reuse – the focus is flow control.

Why choose plastic detention tanks

Concrete and steel once dominated the category, but high-strength polyethylene has changed the game for residential and light-commercial builds. Benefits include:

• Lightweight – a 3 000 L PE tank can be manoeuvred by two people, cutting cranage costs.
• Corrosion-proof – plastic resists aggressive soils and saline ground water in coastal regions.
• Flexible design – factory-moulded chambers, modular crates and pillow bladders suit tight footprints.
• Lower embodied carbon than cement-based equivalents.
• Smooth internal walls reduce sediment build-up, supporting long service life.

Key specification checklist

• Volume requirement (usually 1–5 % of impervious catchment).
• Peak outflow rate (L/s) mandated by the consenting authority.
• Available footprint and cover depth.
• Load class: light vehicle, heavy vehicle or landscaped only.
• Connection size & orientation for inlets, outlets and overflows.
• Maintenance access – 600 mm manhole or larger.

Sizing & compliance in New Zealand

Councils reference NZ Building Code clause E1 Surface Water alongside local stormwater bylaws. In Christchurch, post-quake intensification has made peak flow limits especially strict, pushing demand for plastic detention tanks that can slip under driveways and decks. Work with your hydraulic engineer to model catchment areas and detention time, then confirm the orifice plate size that delivers the required discharge curve.

Need a broader overview of stormwater solutions? See our Stormwater Management hub.

Installation tips

Above-ground vs underground plastic detention tanks

• Above-ground options are fast to position and allow easy inspection, but they can be visually prominent and may need seismic restraint.
• Underground tanks free up valuable outdoor space and protect water from UV; however, they require careful bedding, backfilling and buoyancy control in high-water-table sites.

Always follow the manufacturer’s excavation guide. Use clean, compactable sand or GAP 7 as backfill and avoid point loading on pipe penetrations. Before final sign-off, flush the tank to ensure the orifice isn’t obstructed by construction debris.

Cost factors & procurement steps

Plastic detention tanks range from ~$800 for a 1 000 L above-ground unit to $10 000+ for multi-module underground cells complete with flow-control pits. Budget for:

• Hydraulic design and consent fees.
• Excavation, bedding material and backfill cartage.
• Connectors, vents and debris screens.
• Flow-control devices (orifice plate or vortex valve).
• Ongoing inspection and cleaning every 12–24 months.

Ready to firm up numbers? Compare specs across our leading brands, then click “Get a quote” to connect directly with suppliers for current lead times and delivery options.

Complementary stormwater products are also available, including channel drainage for hardscape run-off, flood protection barriers, storm drain baskets for debris capture and stormwater drainage pipe systems.