How NSW Builders Can Get Rainwater Tank Sizing Right

How NSW Builders Can Get Rainwater Tank Sizing Right 

Rainwater tanks are no longer a “nice extra” on NSW residential projects. With BASIX, stricter NatHERS targets and increased focus on sustainability, correctly sized rainwater tanks are now a compliance issue and a potential source of disputes if they don’t perform as promised.

As a CPD provider for NSW builders, we see the same questions again and again:

• How big should the tank actually be?

• How do I explain the logic to clients and certifiers?

• What are my legal obligations if the tank system underperforms or isn’t installed as per BASIX?

This blog walks you through the essentials of rainwater tank capacity calculation and the practical checklists every NSW builder should use on site. It’s based on our 1-hour CPD session for NSW builders and is designed to help you lift compliance, reduce risk and improve your documentation trail.

Why rainwater tank sizing matters for NSW builders

On typical residential projects, the required tank size and connected roof area are locked in through the BASIX certificate. Those water commitments then become conditions of consent, just like setbacks or floor space ratio. If the tank is too small or the roof catchment isn’t as specified, the project may technically be non-compliant.

Add to this:

• NCC / Plumbing Code of Australia (PCA) and AS/NZS 3500 requirements for backflow, overflows and pipework

• Local planning controls on tank siting, height and visual impact

• Statutory warranties under the NSW Home Building Act

…and it’s clear why rainwater tanks should sit inside your risk management and QA system, not just be left to the plumber on the day.

Good news: with a simple method and a few structured checklists, you can control this risk and show that you acted reasonably and followed an appropriate process.

A simple method to calculate rainwater tank capacity

On BASIX projects, the tool tells you the minimum tank capacity and connected roof area. But it’s still useful to understand the underlying logic so you can sanity-check designs, discuss options with clients and apply the method on non-BASIX jobs.

Here’s a straightforward approach you can use.

1. Work out effective roof catchment

Start with the roof plan and calculate the horizontal area that drains to the tank:

• Measure (or take off from drawings) the roof sections that feed the nominated gutters and downpipes.

• Exclude decks or other areas that are not connected.

Call this area A in square metres (m²).

2. Find the site’s annual rainfall

Use long-term average rainfall data from the Bureau of Meteorology or council resources for the project suburb.

Call this R in millimetres per year (mm/year).

3. Apply a runoff coefficient

Not all rainfall ends up in the tank; some is lost through splash, evaporation and first-flush devices. For a typical metal roof with gutters, a runoff coefficient between 0.8 and 0.9 is commonly used.

Call this C (use 0.85 as a sensible default).

4. Estimate annual harvest

Now you can estimate how much water the roof can deliver to the tank each year:

$$Y = A \times R \times C$$

Where Y is the annual harvest in litres/year (because m² × mm gives litres).

Divide by 365 to get an average daily harvest.

5. Estimate daily non-potable demand

Next, estimate how much rainwater the household will actually use for non-potable fixtures, typically:

• Toilets

• Washing machine

• External garden taps

You can use simple per-person allowances, for example:

• Toilets: ~20–25 L/person/day

• Laundry: ~25–35 L/person/day

• Outdoors: 20–40 L/day for a typical garden

For a 4-person household, a working estimate of around 50 L/person/day for non-potable use (≈ 200 L/day in total) is reasonable.

6. Decide if the site is rainfall-limited

Compare the average daily harvest with the daily demand:

• If harvest < demand, the site is rainfall-limited. The tank will often run low between rain events, so storage becomes less critical than expectations management.

• If harvest > demand, the site has ample rainfall, and the tank is mainly about buffering dry periods.

7. Choose a storage volume

For most NSW suburban homes, a storage period of 2–4 weeks is a practical starting point. Multiply the estimated daily demand by your chosen number of days, then add 10–20% as a buffer and round to the nearest available tank size (e.g. 3000 L, 5000 L, 7500 L).

Finally, check your chosen size against the BASIX minimum. You can always go bigger than BASIX, but never smaller.

Using checklists to manage risk and compliance

Understanding the maths is one side of the coin. The other side is consistent documentation. This is where builders often get caught out in disputes and audits.

Our CPD course for NSW builders is built around three practical checklists plus a calculation sheet you can use on every project.

1. Design and documentation checklist

During planning and pre-construction, a design checklist helps you confirm:

• BASIX tank capacity, catchment area and connected fixtures

• That the tank position works with setbacks, heights and visual impact rules

• Structural requirements for slabs, bases or underground tanks

• Coordination with services (stormwater, sewer, gas, NBN, power)

• Backflow prevention and overflow details as per PCA / AS 3500

• Client expectations around what the tank will and won’t do

This checklist can sit inside your existing QA system so that rainwater tanks are treated like any other regulated building element.

2. Installation checklist

On-site, an installation checklist gives your supervisors a clear list of items to inspect and photograph, such as:

• Base preparation and anchoring

• Inlets, leaf guards and first-flush devices

• Overflow sizing and lawful point of discharge

• Correct labelling and separation of non-potable pipework

• Pump location, protection and electrical isolation

• Confirmed connections to toilets, laundry and external taps

This isn’t just about quality; it’s a record that you have actively checked what the plumber has done.

3. Commissioning and handover checklist

In many disputes, the homeowner claims they “were never shown” how their system works. A simple handover checklist can close this gap by recording that you:

• Tested pump operation and rain/mains switching

• Checked for leaks and correct overflow performance

• Provided manuals, warranties and maintenance guidance

• Walked the owner through the location of valves, switches and filters

Signed checklists, combined with date-stamped photos, can be invaluable if the matter ever reaches Fair Trading or NCAT.

Benefits for NSW builders: fewer defects, stronger documentation

When you combine a clear capacity calculation method with structured checklists, you gain several advantages:

• Lower risk of non-compliance with BASIX, NCC and plumbing standards

• Stronger position in disputes, because you can show a systematic process

• More consistent outcomes across supervisors and projects

• Better communication with clients, who understand what their tank can realistically deliver

For builders operating in a competitive NSW market, this can be a real point of difference—and it directly supports your obligations under the Home Building Act.

Want to go deeper? Join our free CPD program for NSW builders

If you found this overview useful, our full 1-hour CPD session for NSW builders takes you through:

• Detailed worked examples of tank sizing

• Common compliance traps on real projects

• Downloadable PDFs: rainwater tank capacity calculation sheet and three checklists (design, installation, handover)

• Practical tips on how to document your process for certifiers and insurers

We run this as part of our formal (non-RTO) CPD program for NSW builders, with digital certificates you can keep for your CPD records. Places are limited because we provide templates and support, so if you’d like to strengthen your systems around rainwater tanks, now is a good time to jump in.