Supplementary water, formerly known as off-allocation water, is effectively surplus flow that cannot be captured, or ‘re-regulated’, into storages. When storm events result in flows that cannot be captured (regulated) in storage structures such as dams or weirs for future use, and the water is not needed to meet current demands or commitments, then it is considered surplus to requirements. Regulated rivers become unregulated for a period of time.
As soon as these conditions are identified for a particular river, a period of Supplementary Access is announced and details of the river reaches and time periods for supplementary access are published. Supplementary water access licence holders can only pump water against these licences during these announced periods. Other categories of licence holders can choose to pump water during these periods as usual.
Those holding general security water access licences may, under some circumstances, and in select valleys, pump water 'without debit' during these periods. For more information on pumping during supplementary flow events go to the relevant water sharing plan.
Supplementary flow events can occur in any regulated system at any time and therefore access is purely opportunistic. Supplementary events depend on the amount and location of rainfall and ensuing streamflow, and the catchment conditions at the time. They can be triggered overnight and last for a day or two, a month to six months or more, depending on the river system and nature of the flow event.