Rainwater Harvesting and Reuse Systems
What You Need to Know
When BASIX applies to your project in NSW, you will almost certainly need a rainwater tank. BASIX targets a 40% reduction in potable water use against the NSW benchmark of 247 litres per person per day. Rainwater harvesting is the most common way to hit that target. AS/NZS 3500.1 sets the plumbing rules for how the tank connects to the building. The NCC Plumbing Code (Parts B6 and B7) sets the performance requirements for collection, storage, and reuse. Get the tank size wrong and you either waste money on a tank that sits half-empty, or you fall short on your BASIX score.
The Rules
- BASIX requires a water reduction target of 40% below the NSW benchmark for new residential developments (BASIX, State Environmental Planning Policy)
- A rainwater harvesting system must only collect roof water (NCC Volume Three, Part B6)
- No unprotected cross-connection is permitted between the rainwater system and the drinking water supply (AS/NZS 3500.1:2025, Section 16)
- Top-up lines from the mains water supply must include a backflow prevention device rated to the hazard (AS/NZS 3500.1:2025, Section 4)
- All rainwater pipes, outlets, and tanks must be clearly labelled as non-drinking water, unless rainwater is the sole supply on the property (AS/NZS 3500.1:2025, Clause 16.3.2)
- Hot water systems using rainwater must store water at 60 degrees C minimum to prevent Legionella (AS/NZS 3500.4:2025)
- NSW Health supports rainwater for toilet flushing, laundry, hot water systems, garden watering, car washing, and pool filling - but does not recommend it for drinking where mains water is available
- Tanks over 10,000 litres may require separate council approval depending on the local planning instrument
What This Means in Practice
Take a new 20-unit residential flat building in western Sydney. The site has a roof catchment area of 600 m². The Bureau of Meteorology shows average annual rainfall of about 800 mm for the area.
Step 1: Calculate Annual Yield
600 m² × 800 mm × 0.85 (runoff coefficient for a metal or concrete roof) = 408,000 litres per year.
Step 2: Estimate Demand for Connected End Uses
Each unit has a toilet (average 30 litres per person per day) and a washing machine (average 40 litres per person per day). With an average of 2 people per unit:
- Toilets: 30 L × 2 people × 20 units = 1,200 L/day = 438,000 L/year
- Laundry: 40 L × 2 people × 20 units = 1,600 L/day = 584,000 L/year
- Combined demand: 1,022,000 L/year
The annual yield of 408,000 litres does not cover full toilet and laundry demand. This is normal. BASIX does not expect 100% offset. It credits the proportion of non-potable demand that rainwater can serve. A mains top-up line fills the gap during dry spells.
Step 3: Size the Tank
A common rule is to hold about 4 weeks of supply. At 408,000 L/year, the daily average yield is about 1,118 L/day. Four weeks of storage = 1,118 × 28 = roughly 31,000 litres. In practice, the designer runs BASIX modelling to find the point where a larger tank stops improving the score. For this project, a 20,000 litre tank connected to toilets and laundry across all 20 units is a realistic starting point. Going to 30,000 litres might add 2–3 more BASIX points but costs another $5,000–$8,000 in tank and pipework.
Step 4: Specify the System
The tank sits at ground level (or underground if space is tight). A pump draws water from the tank and pressurises a dedicated non-potable riser. A mains top-up with a backflow prevention device fills the tank when it drops below a set level. A first-flush diverter sends the first 20–40 litres of roof runoff per 100 m² to the stormwater drain, keeping leaves, dust, and bird droppings out of the tank. Overflow connects to the site stormwater system per AS/NZS 3500.3.
Key Design Decisions
Tank Size vs. BASIX Score
Run the BASIX tool with different tank sizes to find the sweet spot. There is always a point of diminishing returns where a bigger tank barely lifts the score. For most multi-residential projects, the practical range is 10,000–30,000 litres. Single dwellings typically need 2,000–5,000 litres.
Connected End Uses: Toilets Only vs. Toilets and Laundry
Connecting both toilets and laundry uses more rainwater and scores better on BASIX. But it also means running a second non-potable riser through the building and plumbing two services to each unit instead of one.
Above-Ground vs. Underground Tank
Above-ground tanks are cheaper to install and easier to inspect. Underground tanks save space, sit out of sight, and work well under driveways or landscaped areas. Underground tanks need waterproof access hatches and pump chambers.
Mains Top-Up Arrangement
A trickle top-up keeps the tank partly full at all times. An automatic solenoid valve with an air gap or reduced pressure zone device prevents backflow into the mains. The air gap method is the simplest and most reliable, but it needs physical space above the tank.
Who Needs to Know What
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References
- AS/NZS 3500.1:2025, Plumbing and drainage — Part 1: Water services (Section 16: Rainwater tanks and rainwater supply systems)
- AS/NZS 3500.3:2025, Plumbing and drainage — Part 3: Stormwater drainage
- AS/NZS 3500.4:2025, Plumbing and drainage — Part 4: Heated water services
- National Construction Code 2022, Volume Three, Part B6 — Rainwater services
- National Construction Code 2022, Volume Three, Part B7 — Rainwater storage
- BASIX (Building Sustainability Index), NSW Department of Planning and Environment - Rainwater Harvesting Systems
- BASIX Rainwater Harvesting System Guidelines, NSW Government