Is a first flush diverter required on a rainwater tank?
Summary (TL;DR)
While AS/NZS 3500.1 strongly recommends the use of first flush diverters to improve water quality, they are often made mandatory by local council development codes or state health guidelines, especially if the rainwater is plumbed into the house for toilets or laundry.
Rainwater Tank Requirements
When it rains, the first few millimetres of water wash dirt, bird droppings, leaves, and pollution off the roof. A first flush diverter captures this dirty water and prevents it from entering the rainwater tank.
What the Standard Says
AS/NZS 3500.1 covers the installation of rainwater harvesting systems. It mandates that all inlets to the tank must be protected by a leaf strainer / mesh screen to prevent mosquitoes and vermin from entering. While it highly recommends first flush diverters, it often defers to local authorities for mandatory requirements.
Local Council Rules
In Queensland, if a rainwater tank is plumbed internally (e.g., to supply toilets and washing machines), most local councils (like Brisbane and Gold Coast) require a first flush diverter to be installed as part of their health and development codes.
Sizing the Diverter
A general rule of thumb for sizing a first flush diverter is to capture 0.5 to 2 litres of water per square metre of roof area, depending on the level of pollution in the area (e.g., lots of overhanging trees require a larger flush).
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