Rainwater Tanks for New Homes – BASIX Compliance & Sizing 2025

Rainwater tanks are one of the easiest ways to meet water efficiency targets, reduce mains water bills and improve sustainability scores in new Australian homes. In NSW especially, they are often the key to passing BASIX (Building Sustainability Index) requirements for new dwellings.

This guide covers the national rules from the Plumbing Code of Australia (PCA) and AS/NZS 3500.1:2025 Clause 9 (Rainwater services), plus NSW-specific BASIX wins for rainwater reuse to toilets, laundry, washing machines and garden taps in 2025/2026.

01National Rules – Rainwater Reuse Across Australia

The Plumbing Code of Australia (PCA) Part B6/B7 and AS/NZS 3500.1:2025 Clause 9 allow and regulate rainwater harvesting and reuse nationwide — the same rules apply in NSW, VIC, QLD, SA, WA, TAS, NT and ACT.

Rainwater can be used for:

• Toilet flushing
• Laundry (washing machines & tubs)
• Garden irrigation / watering
• Outdoor / garden taps
• Car washing
• Other non-potable uses (e.g., cooling towers if approved)

Prohibited for drinking, food preparation, personal hygiene (showers/basins).

• Complete separation — Rainwater pipes/tanks/outlets must never connect to drinking water supply (no cross-connections).
• Backflow prevention — RPZD or equivalent at any potential connection point (Clause 8).
• Labelling — All pipes, taps and outlets must be clearly marked "NON-DRINKING WATER – DO NOT DRINK" (Clause 5.8.2). Use green colour coding or permanent labels.
• Overflow — Tank overflow must discharge to stormwater system (not sewer).
• Tank design — Screened inlet, leaf filter, first-flush diverter, access for cleaning (Clause 9.3).
• Pumps (if used) — Installed with backflow protection (Clause 9.4).

02When Rainwater Reuse Is Mandatory (or Effectively Required)

Rainwater tanks/reuse is not compulsory for every home, but it becomes mandatory or unavoidable in these common job types:

• New homes in NSW— BASIX (Building Sustainability Index) requires water savings targets (40–50% reduction). Rainwater tanks for toilets + laundry + garden taps are one of the easiest ways to meet the score.
• VIC – Melbourne Water / council WSUD policies— Many new subdivisions require rainwater tanks for non-potable reuse (toilets, laundry, irrigation).
• QLD – Water Sensitive Urban Design (WSUD)— Councils (Brisbane, Gold Coast) often mandate tanks for new developments to reduce mains demand.
• Multi-unit / apartments (Class 2)— Roof space makes rainwater harvesting economical; many body corporates and councils require it.
• NDIS SDA (Class 3) & aged care— Sustainability and cost-saving targets often push rainwater reuse for toilets/laundry.
• Bushfire-prone areas— AS 3959 requires static water supply (tanks); dual-use for toilets/garden is common.
• Green Star / 7-Star NatHERS homes— Rainwater reuse scores points for water efficiency.

03NSW-Specific – Rainwater Tanks & BASIX Compliance 2025

In NSW, rainwater tanks are not mandatory for every new home, but they are often the easiest way to meet BASIX water savings targets (typically 40–50% reduction from mains water use).

• New homes must achieve a BASIX water score (calculated online at basix.nsw.gov.au).
• Rainwater tanks score points for supplying toilets and laundry (highest impact) + garden taps.
• Typical tank size to pass BASIX comfortably: 2,000–5,000 L (connected to toilets + laundry + garden taps).
• First-flush diverter and leaf screens boost points (improves water quality).
• Rainwater reuse often saves 40–60% of toilet/laundry demand — enough to meet most BASIX targets without expensive water-efficient fixtures.

• New subdivisions or large developments (e.g., Inner West, Parramatta, Blacktown) — council DA conditions often require tanks.
• BASIX pass — if your design scores low on other measures (e.g., no low-flow taps), a rainwater tank is usually the fix.

04Rainwater Tank Size Calculator (Quick Guide)

Use this rough guide based on AS/NZS 3500.1 Clause 9 and BASIX typical outcomes:

• Small house (2–3 bedrooms, 2 toilets): 2,000–3,000 L.
• Medium house (3–4 bedrooms, 3 toilets): 3,000–5,000 L.
• Large house (4+ bedrooms, 4+ toilets): 5,000–10,000 L.

Connect to: Toilets (highest savings) + laundry + garden taps.

05Practical Estimating Checklist for Sydney Jobs

• Tank: 3,000–5,000 L slimline or round (Reece stocks Kingspan, Graf, Tankworks)
• Pipes: Dedicated green poly or PVC to toilets/laundry/garden taps
• Backflow: RPZD at mains connection (high-hazard)
• Labelling: Permanent "NON-DRINKING WATER" stickers on all taps/pipes
• Cost allowance: $3,000–$12,000 (tank + pump + pipes + install) — often offset by BASIX points or council incentives

Rainwater reuse is national, legal, and highly beneficial — especially in NSW new homes where BASIX makes it almost essential.

At SNZ Plumbing Estimating, we include rainwater tanks and compliant reuse systems when required — accurate quotes, full BASIX compliance.

• free bedding and spoil calculator
• civil and stormwater estimating
• how to calculate bedding and spoil
• why the cheapest estimate costs you most

More on codes and compliance

06Related reading

• Bushfire-Resilient Plumbing: New Clauses in NCC B4D6/B6D6 and AS/NZS Impacts – For High-Risk Areas, with Estimating Tips for Materials
• Backflow Prevention: When RPZD Is Required & Tundish Setup
• NDIS SDA Plumbing Requirements – Class 3 Dwellings 2025 Guide
• How to Size Hot Water Systems Correctly – Avoid Cold Showers in Large Homes

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