Rainwater Tanks and BASIX: Integrating Roof Drainage in NSW

BASIX Makes the Tank Part of the Drainage System

Almost every new home in NSW carries BASIX water commitments, and for most projects the practical answer is a rainwater tank plumbed to toilets, laundry or irrigation. BASIX began under its own State Environmental Planning Policy and now operates within the NSW sustainable buildings planning framework — but for builders the operative fact is unchanged: the BASIX certificate is part of the approval, its commitments are checked at certification, and the tank is not a landscaping item. It is a hydraulic component of the roof drainage system, and the drainage plan has to treat it that way.

What the BASIX Certificate Actually Commits You To

The certificate generated for your project typically specifies:

• A minimum tank volume — determined by the BASIX assessment for your specific dwelling, not a standard number. Do not assume a size; read the certificate.
• A minimum connected roof area — the share of roof that must drain to the tank for the water savings to be real.
• The reuse connections — which fixtures (commonly toilets, laundry cold water and outdoor taps) the tank must supply.

The certifier checks these commitments before issuing the final occupation certificate, and mismatches — a smaller tank substituted on site, or half the committed roof area never actually piped to the tank — are a classic cause of last-minute certification grief.

Designing the Roof Drainage Around the Tank

Integrating the tank changes the roof drainage layout in ways that should be resolved on the drawings, not improvised by the roof plumber:

• Catchment selection. The connected roof area must be physically collectable to the tank location. That usually means grouping downpipes on the tank side of the house, or running charged (wet) lines from remote downpipes under the slab or paving to the tank — which brings its own design rules: the tank inlet below the gutter outlets, pressure-rated buried pipework, and a drain-down point.
• Sizing still follows AS/NZS 3500.3:2021. Gutters, downpipes and the collection pipework are sized for the design rainfall as usual — the tank does not reduce the design flows, because it must be assumed full in a storm.
• The overflow is a real pipe. When the tank is full — which is precisely when it rains hardest — the entire connected catchment discharges through the tank overflow. The overflow must match the inlet capacity and be piped to the lawful point of discharge, exactly like any other downpipe. An overflow dribbling onto the ground beside the slab fails inspection and undermines footings.
• Screened inlets and overflows for mosquito protection, and first-flush or leaf management where the tank supplies internal fixtures.

Tanks, Detention and the Council Layer

A BASIX reuse tank is not the same thing as on-site detention. A reuse tank is assumed full and provides no reliable storm storage; OSD must be empty and available when the storm arrives. Where a council DCP requires detention, some councils accept combined tanks with a dedicated detention zone above the reuse volume and a controlled discharge — but the rules differ between councils, so confirm the current DCP before assuming a single tank can do both jobs. On small infill lots, this distinction frequently decides whether one tank or two structures are needed.

Practical Tips for Builders

• Read the BASIX certificate at tender stage — tank volume, connected roof area and reuse connections all carry cost.
• Locate the tank early. Access for the slimline tank behind the garage, the charged line route, and the overflow path to the street are layout decisions, not afterthoughts.
• Keep the drainage plan and BASIX consistent. The stormwater plan should label the connected roof area and show the tank, pump, reuse points and overflow so the certifier can tick the commitments off one document.
• Don’t substitute the tank without checking. Changing tank size or connected area on site can require an amended BASIX certificate before the occupation certificate.

One Plan That Satisfies BASIX, the NCC and Council

Contrive Consultants prepares council-ready stormwater and roof drainage plans across Sydney and NSW that integrate BASIX rainwater tanks, charged lines and overflows into a single certifiable package. See our services or contact us on +61 497 848 111 or info@contriveconsultants.com.au.