Victoria has officially made its largest-ever desal water order.

An order of 125 gigalitres of water from the Desalination Plant in Wonthaggi has been announced for the 2019/20 year.

The order is designed to boost and flush out storages after an extremely dry spring and summer.

The large order for the plant is needed to prevent Melbourne’s water storages dropping to a point where they pose a significant risk to water security.

Melbourne’s water storage levels recently dropped to less than 55 per cent and would be below 50 per cent if not for the previous desalination orders.

The order is designed to protect Victoria’s water supply as the state experiences its lowest water levels since the Millennium Drought in 2011.

The authorities say supplies have been smashed by an extreme dry season, the effects of climate change, the threat of bushfires to catchment areas, and a rising population.

Melbourne’s water storages have had below average rainfall inflows in 18 of the past 20 years while population has grown by 1.4 million.

The cost to customers for this essential order has been minimised to an estimated $10 across a year’s worth of bills.