Rainwater Tank Installation on Residential Builds in Australia

What it is

A rainwater tank collects roof runoff for non-potable or potable reuse inside a dwelling. On a new residential build the tank is often a BASIX or 7-star NatHERS requirement rather than a homeowner choice. The plumbing connection, tank sizing and overflow path all sit under AS/NZS 3500.1 and HB 230, with state and council variations layered on top.

Builders carry the design risk here. A tank that meets the engineer drawing but fails council inspection for backflow protection or overflow discharge is a rework cost the head contractor wears.

Governing documents

AS/NZS 3500.1 sets the plumbing rules for cold water services including rainwater tanks and rainwater supply systems. The 2025 edition is current in most states. HB 230 Rainwater Tank Design and Installation Handbook sits alongside the standard and gives practical guidance on sizing, first flush diverters, screening and serviceability.

The National Construction Code references AS/NZS 3500.1 in Volume Three, the Plumbing Code of Australia. State plumbing regulators then call up the NCC by reference. The chain runs from building permit, to plumbing permit, to the standard, to the handbook.

Sizing and supply

Tank sizing follows from the demand it supplies and the catchment area. A typical Class 1 dwelling in Sydney with a 200 square metre roof can supply toilet flushing and laundry cold water from a 5,000 litre tank for most of the year. Garden irrigation and clothes washing push the demand higher and sometimes require larger storage or a mains backup.

BASIX in NSW assigns a points value to the tank based on capacity and the fixtures it supplies. Victorian builds use the 7-star NatHERS rating plus the Plumbing Regulations which require rainwater connection to toilets in many new homes. Queensland Development Code MP 4.2 sets minimum standards for new houses including a rainwater tank or alternative water source.

Plumbing connection rules

A tank supplying internal fixtures must connect through approved fittings, with non-potable signage at every outlet and pipework identified by colour or marking. AS/NZS 3500.1 sets the labelling requirements. The supply line to toilets and washing machines runs separately from the potable main and cannot cross connect.

A mains backup is needed where supply security matters. The mains top up either flows into the tank through an air gap or connects through a testable backflow prevention device. AS/NZS 3500.1 Table 4.4.1 lists the acceptable devices for the risk level. Above ground tanks with a mains top up by air gap are low hazard. Buried tanks with a direct mains connection are high hazard and need a registered backflow device tested annually.

Backflow protection

Backflow protection sits at the boundary of the rainwater system and the drinking water main. The risk is rainwater being pulled into the potable supply during a pressure drop. A dual check valve handles low hazard situations. A reduced pressure zone device handles high hazard. Above ground tanks usually fall in the low category. Below ground tanks fall in the high category.

The Water Authority issues the rules for backflow registration and testing. Sydney Water, Yarra Valley Water and Unitywater all maintain installer registers and inspection schedules.

Overflow and stormwater

Tank overflow discharges to the legal point of stormwater discharge. That is usually the council stormwater main, a soakwell on the lot, or a level spreader to garden. The overflow pipe is sized at least one size larger than the inlet to handle storm intensity. Mosquito proof screens at the inlet and overflow are required by HB 230 and most state guidelines.

Council approval

Most councils accept the plumbing permit for a rainwater tank below a certain volume as the full approval. Larger tanks or those on sloped sites can need a separate Development Application for siting. Setbacks from boundaries, neighbour overshadowing and bushfire attack level rating all influence tank placement.

Get the BASIX, 7-star or QDC paperwork in order before lodging the plumbing permit. Councils refuse plumbing permits if the energy and water compliance document does not match the tank schedule.