Backflow Prevention : an overview

Here you can find out how backflow occurs and how a containment device will protect water supplies.

Backflow is a reversal of the normal direction of water flow in your water supply plumbing system. It presents a hazard where your water supply is inadvertently interconnected with a source of pollution. Those pollutants could then enter the water supply.

Backflow can only occur when the water delivery main is at a lower pressure than your plumbing system. For example, when there is a burst or ruptured main; or excessive demand during fire fighting operations and the like; or if a pump is connected to your water plumbing system.


Preventing Backflow Contamination

Water Corporation policy requires each and every connection to the drinking water supply (including fire services) to have fitted a backflow prevention device at the boundary that is appropriate to the high, medium or low risk hazard posed by the activities carried out at the property.

The policy is applied at the building application stage. The following application types will be assessed for backflow prevention risk:

  • New developments, or redevelopments of existing facilities;
  • Changes to any existing water service;
  • Additions / Alterations (which also includes where no water service applications are included, but only if the property has existing meters);
  • Multiple Residence and/or Commercial (which also includes where no water service applications are included, but only if the property has existing meters);
  • Any applications that involve a fire service;
  • Any applications where the minimum service size applied for is greater than 25mm service size (except fire services – where the advice applies regardless of the service size).

Refer to the policy page for more information.

Backflow prevention devices at the boundary be owned by the property owner and adequately maintained by the owner/occupier to ensure on-going protection of the drinking water supply. High and medium level devices require testing at commissioning, after any maintenance or at least annually to ensure correct operation.

Some forms of backflow prevention will reduce water pressure/flow rate down stream of the device. If water pressure and flow rate is critical for the activities on a property then the property owner needs to consult with a licensed plumber or hydraulic consultant prior to choosing and installing the final form of boundary backflow protection.

Property owners are responsible for providing zone and individual backflow prevention protection from hazards within their property as specified in AS/NZ 3500.1:2003.

Only licensed plumbers can install, commission and test backflow prevention devices.

Backflow Definitions

Air gap * In a water supply system it is the unobstructed vertical distance through the free atmosphere between the lowest opening of a water service pipe or fixed outlet supplying water to a fixture or receptacle and the highest possible water level of such fixture or receptacle.
Backflow prevention device * A device to prevent the reverse flow of water from a potentially polluted source into a potable water supply system.
BuilderNet® An online system for lodging all types of building applications to the Water Corporation for quotation and approval.
By-laws The Metropolitan Water Supply, Sewerage and Drainage By-laws 1981 (the MWSS&D By-laws) or the Country Areas Water Supply By-laws 1957 (the CAWS By-laws) as appropriate and as amended.
Containment The fitting of an approved backflow prevention device on the water service connection to a property. It is part of the plumbing system for that property and contains any potential contamination from that property to the property. Also commonly known as boundary protection.
Cross-connection * Any connection or arrangement, physical or otherwise, between any potable water supply system either directly or indirectly connected to a water main, and any fixture, storage tank, receptacle, equipment or device through which it may be possible for any non-potable, used, unclean, polluted or contaminated water, or any other substance, to enter any part of such potable water system under any conditions.
Double-check valve * A device to prevent backflow caused by backpressure, and which has two independently operating force loaded non-return valves and incorporates specific test points for in-service testing.
High risk * Any condition, device or practice that, in connection with the drinking water supply system, has the potential to cause death.
Individual protection The fitting of a backflow prevention device at the point of potential contamination.
Low risk * Any condition, device or practice that, in connection with the drinking water supply system, constitutes a nuisance but does not endanger health or cause injury
Medium risk * Any condition, device or practice that, in connection with the drinking water supply system, has the potential to endanger health.
Plumbing regulations The Water Services Licensing (Plumbers Licensing and Plumbing Standards) Regulations 2000, as amended and administered by the Plumbers Licensing Board.
Zone protection The fitting of a backflow prevention device so that it isolates an area or building within the property.

* These definitions are in accordance with the joint Australia/New Zealand Standard AS/NZS 3500.