Backflow Prevention

Backflow Prevention Systems – Technical Information

One of the biggest risks to a healthy water supply is backflow. It can be a source of contamination that can seriously affect the quality and safety of our drinking water. This can lead to sickness and in some cases death.

How Backflow Happens


Backflow can only occur where there is a connection or cross-connection to the main water supply. Cross-connections can occur in any situation where fixtures are connected directly to the main supply such as:

  • irrigation systems
  • dishwashers
  • washing machines
  • coffee machines
  • swimming pools, spa pools or ornamental pools that are filled by hose
  • water softeners
  • pesticide and fertiliser attachments for hoses
  • fridges and icemakers
  • bidets
  • retractable spray outlets to tubs and sink
  • flexible shower hoses
  • storage tanks.

Backflow is caused when the water pressure drops in the water distribution system and this causes water to flow backward into the public water supply network. Water is normally maintained at a significant pressure to enable water to flow from taps, showers, and other fixtures. The water pressure may fail or be reduced when there is unexpectedly high demand on the water system Reduced pressure in the pipes can allow contaminated water to be drawn up into the system.

Preventing Backflow


Preventing water contamination from backflow is easy. A boundary backflow prevention device at the service connection/s needs to be installed and you need to have the device tested annually. This device, when maintained and working properly, will trap water that begins to flow the wrong way and prevent contamination of the public water supply.

Legal Requirements


Building Code Clause G12 Water supplies requires that potable (drinkable) water supply must be protected from contamination and installed in a manner that avoids the likelihood of contamination within the system.

Acceptable Solution G12/AS1 requires backflow prevention to be provided where it is possible for water or contaminants to backflow into a piped potable (drinkable) water supply. Backflow can be prevented either through an air gap or a backflow prevention device.

The Acceptable Solution also provides that there must be no likelihood of a cross-connection between a private water supply (such as a rainwater tank) and mains water supply.

The responsibility for preventing backflow may rest with:

  • the network utility provider who may install a backflow prevention device as part of the meter assembly, or
  • the individual property owner whose responsibility it is to comply with the requirements of the network utility provider and the Building Code, and to protect building users.


The responsibility for preventing backflow starts at the water meter and includes your entire property’s water system. If you use substances on your property that could enter the public water supply and then be consumed causing death or injury, it is your legal obligation to install a boundary backflow prevention device. Each year you must get your backflow device tested. If your device fails the annual test, you will need to make the necessary repairs and submit the certified documentation. Hamilton Plumbing tradesmen can assist with this.

Property Classification

It is important that the correct boundary backflow prevention device is in place to contain the degree of risk of backflow on your property. Three degrees of risk are used to classify properties in order to determine the type of backflow device required.

  1. High Risk sites are properties that use or produce products of a toxic or bacterial nature that may cause death or serious illness if leaked into the main water supply. Examples of high risk sites are hospitals, mortuaries, chemical plants, laboratories, cooling towers, hairdressing salons, food processing and other manufacturing plants. Residential homes with irrigation systems (pop-up sprinklers), swimming pools and spa baths are also considered high risk.

Backflow prevention device: Reduced pressure zone device (must be tested annually).

  1. Medium Risk sites can endanger health if backflow occurs. Examples of medium risk sites are public swimming pools, garden irrigation systems, drink dispensers with carbonators, commercial laundries and rainwater tanks connected to household plumbing.

Backflow prevention device: Double check valve (must be tested annually).

  1. Low Risk sites are properties that could cause a nuisance by colour, odour, or taste but do not endanger health. An example of a low risk site is a residential home with only domestic water use. Backflow prevention device: Air gap separation or hose vacuum break dual check valve (these devices do not require testing; they only require certification that have been installed correctly).

How we can assist

Hamilton Plumbing has the technology and experience to perform testing on your current water supply, perform all repairs and maintenance checks to your current backflow prevention device and can provide complete installations on new systems.  We are fully licensed and insured and our tradesmen are fully qualified and experienced for your total peace of mind.


Our Backflow Prevention services include:

  • Assessment and advice on your backflow prevention requirements.
  • Annual Backflow Prevention testing – to ensure your device continues to be compliant.
  • Repairs and maintenance to existing devices.
  • Complete full installations of new backflow prevention devices

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