Mains water · Sydney Water
Backflow, and the second water supply on a block
Backflow is used water from a property flowing backwards into the mains, and Sydney Water requires suitable backflow containment on every connection to its water mains. On a block that also has a tank or another water source, what that containment has to be depends on the other source: an above-ground rainwater tank is low hazard, while stormwater or greywater used as an alternative supply is high hazard.
A guide published by Dotto about Richmond, New South Wales, for properties connected to Sydney Water’s mains. General information, not technical advice.
Why a second source matters
Sydney Water says backflow is more likely when the water pressure on a property is higher than in the main, for example when a pump is running on the site, and that contaminants can get in through hoses left submerged in buckets, tanks or pools. A plumbed-in tank usually brings a pump with it: Sydney Water notes that most tanks need one to supply water at the same pressure as the drinking water supply.
The general rules, including the hazard ratings, the device each one needs and the yearly testing of medium and high hazard devices, are set out on Sydney Water’s backflow prevention page. This page covers what changes when a block has more than one water supply.
By the other water source
| On the block | What Sydney Water says |
|---|---|
| An above-ground rainwater tank | Low hazard. With a 20 mm or 25 mm water meter, no additional backflow prevention is needed. |
| A below-ground tank taking rainwater only | A vented dual check valve with an atmospheric port. |
| Tank water running a toilet or washing machine | Assess the site and check the backflow requirements, as a property-specific device may be needed. |
| Stormwater or greywater as an alternative supply | High hazard. A high hazard property needs a registered break tank, a reduced pressure zone device or a registered air gap. |
| A metered standpipe on Sydney Water’s pipes | Only with Sydney Water’s permission, and the standpipe must have a testable double check valve built into its design. |
| A fire hose reel | Must have backflow containment protection, supplied from a Sydney Water metered service. |
When a site’s containment needs are assessed, Sydney Water’s rules for plumbers say the plumber must tell the owner the property’s backflow containment requirements, install a device that matches its hazard rating, and say whether the device needs a yearly test. More on tanks in plumbing a rainwater tank.
Devices inside the house
Containment at the boundary protects Sydney Water’s network. A zone device protects one section of a building’s plumbing, and an individual device protects the connection to one fixture or appliance. Sydney Water doesn’t take test reports for zone and individual devices: questions about those go to Building Commission NSW, which regulates plumbing in NSW.
What Sydney Water checks, and what follows
Sydney Water audits a sample of installations against the Australian Standards AS/NZS 3500.1 and AS/NZS 2845 and its own connection requirements. Where one doesn’t, it sends a notice asking for the device to be repaired, maintained, tested or replaced.
If an owner doesn’t comply with its backflow requirements or notices, Sydney Water says it may:
- test the device and charge for the service
- restrict the water supply
- disconnect the property
- charge any costs it incurs to make the connection meet its requirements.
Where a site’s hazard rating is unknown or has not been assessed, Sydney Water treats the site as high hazard.
Only licensed plumbers with backflow accreditation can inspect, commission and test medium and high hazard devices. Sydney Water keeps a search of accredited backflow plumbers.