Backflow occurs because a condition exists in a water supply system that will cause back-siphonage or back-pressure. Back-siphonage can occur on a property through a vacuum created in the water supply system.
An example of back-siphonage would be a pipeline breakage, undersized pipework or high withdrawal rates. Back-pressure can occur within properties when high pressure is generated downstream by pumps, thermal expansion or elevation.
Backflow prevention starts within a property boundary by isolating a possible contamination source from the town mains or storage tanks (rainwater tanks) from the tap or appliance.