Rainwater Tanks for NSW Homes: Sizing, Rules and BASIX Credit

A rainwater tank is one of the most reliable ways to meet the water side of a BASIX certificate in NSW. But the credit you earn depends on far more than just installing a tank. This guide explains how size, roof area and connections all shape the result.

Why rainwater tanks help your BASIX water target

Every BASIX certificate sets a water reduction target the home must achieve compared with a typical benchmark. A rainwater tank reduces the amount of mains water the household needs, because captured rain is used for tasks that do not require drinking-quality water. The more reliably the tank can supply those uses, the larger the water saving the assessment recognises.

Tanks usually work alongside other measures such as water-efficient fixtures. Together they build up the saving needed to hit the target. Our team can model the combination that suits your design, and you can contact us to discuss the options.

What you can connect a tank to

The value of a tank for BASIX comes from the end uses it serves. Connecting it to high-frequency, year-round uses delivers the strongest credit. Common connections include:

• Toilet flushing. A steady, daily demand that does not need drinking-quality water, making it ideal for tank supply.
• Laundry cold water. Cold-water supply to the washing machine is another consistent use throughout the year.
• Outdoor and garden taps. Useful for irrigation, though demand is seasonal and drops in wet periods.

Connecting a tank to toilets and laundry tends to give a more dependable, year-round saving than garden use alone, because indoor demand does not pause when it rains.

How tank size and roof area work together

Two numbers drive how much water a tank actually delivers: the tank volume and the connected roof catchment area. They have to be balanced.

A large tank connected to only a small slice of roof will rarely fill, so its size is wasted. A small tank connected to a large roof will fill quickly but overflow often, spilling water it cannot store. The aim is a tank big enough to carry water through dry spells, fed by enough roof area to keep refilling it.

Council detention versus BASIX reuse

It is easy to confuse two different tank requirements. They serve opposite purposes and you may need to satisfy both.

• BASIX reuse. This is about capturing and using rainwater to cut mains consumption. The tank needs to hold water and supply it to fixtures.
• Council detention. Some councils require on-site stormwater detention to slow the rate of runoff during heavy rain, protecting downstream drainage. A detention tank needs empty space available to absorb a downpour.

Because reuse wants the tank full and detention wants room to spare, a single tank sometimes has to be designed to do both, with a dedicated detention volume above the reuse storage. Your assessor and a hydraulic consultant can confirm what your council requires.

Practical sizing guidance

The right tank size depends on your roof, your block and your household, so treat the following as general principles rather than fixed figures:

• Connect as much roof as practical. Directing more downpipes into the tank improves how often it refills.
• Prioritise indoor uses. Plumbing the tank to toilets and laundry gives a steadier saving than garden-only use.
• Match the tank to available space. Slimline tanks suit narrow side setbacks on smaller blocks, while round tanks suit open yards.
• Plan for overflow. A clear overflow path to the stormwater system prevents pooling against the house.
• Allow for a pump and switch. A pump with mains backup keeps fixtures supplied when the tank runs low in dry spells.

Getting the balance right early avoids redesigning the plumbing later. Our assessment team can help you settle on a tank that meets the target without oversizing.

Maintenance keeps the credit real

A tank only delivers its saving if it keeps working. Keep gutters and the tank inlet screen clear of leaves, check the first-flush device if one is fitted, and make sure the pump and mains switch are functioning. A neglected tank that no longer supplies the fixtures undermines the very saving it was installed to provide.

Frequently asked questions

Can one tank meet detention and reuse at once? Often yes, but it must be designed with a separate detention volume above the reuse storage. A hydraulic consultant and your council confirm the detail.

Do I have to connect the tank to toilets? Not always, but indoor connections like toilets and laundry give a stronger, year-round water saving than garden use alone, which helps you reach the target.

What size tank do I need? It depends on your roof area, climate and household. There is no single answer, so our team can model the size that suits your project.

Building or renovating in NSW? Get a fast, fixed-fee quote from our HERA-registered assessors.