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water company says i need a backflow device, what is that
The question, put plainly: What is a backflow prevention device and do I need one?
The short answer
A backflow prevention device stops contaminated water being drawn back into the drinking water supply. Water authorities require them at properties with a defined hazard, and most require annual testing by a licensed person.
TradieAnswers editorial · written in-house and sourced to the authorities listed below
Checked against 4 published sources, listed in full at the end of this page. How we source this
If pressure in the main drops, water can flow backwards. Anything connected to the plumbing at that moment, a hose sitting in a pool, an irrigation line, a fire service, a commercial process, can be pulled back into the drinking water supply. Backflow prevention devices are the physical barrier that stops that happening.
What triggers a requirement
- Irrigation systems, particularly those that inject fertiliser.
- Swimming pools and spas with a permanent top up connection.
- Fire services.
- Rainwater tanks plumbed into the house with a mains top up.
- Any commercial or industrial activity on the property.
The ongoing obligation
Testable devices generally have to be tested annually by a person with a backflow prevention endorsement, and the result lodged with the water authority. This catches people out after they buy a property with an existing device, since the obligation attaches to the property rather than following the previous owner.12
Sources
Every source below was fetched and its response recorded on the date shown. Where a government site blocks automated clients, that is noted rather than reported as a successful check.
- Backflow prevention · Sydney WaterChecked 2026-08-10 · responded 200 OK
- NCC 2022 Volume Three: Plumbing Code of Australia · Australian Building Codes BoardChecked 2026-08-11 · responded 200 OK
- Plumbing and drainage licences · Queensland Building and Construction CommissionChecked 2026-08-10 · responded 200 OK
- Building and Plumbing Commission · Building and Plumbing Commission VictoriaChecked 2026-08-10 · live, but blocks automated clients so the specific page could not be confirmed
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