The largest water order in Victorian history will be made from the multi-billion-dollar Wonthaggi desalination plant as the state continues to grapple with dry conditions and plunging water storages.

Key points: The order will be for more than 50 and up to 150 gigalitres. The last order, in 2018, was for 46 gigalitres

The order will be for more than 50 and up to 150 gigalitres. The last order, in 2018, was for 46 gigalitres Melbourne's water storages are currently at 57 per cent

Melbourne's water storages are currently at 57 per cent Dams are being affected by fires, but authorities are confident the impact will be minimal

Water Minister Lisa Neville told the ABC that because of prolonged dry conditions, which are forecast to continue through autumn, the government would make an order of between 50 and 150 gigalitres.

Melbourne's water storages are at just 57 per cent.

The city's largest water source, the Thomson Dam in Gippsland, has been affected by fires on its banks which continue to burn.

Every year Melbourne uses 175 gigalitres of water from the dam.

Officials had been worried that run-off from the fires in the dam's catchment would contaminate water, but Ms Neville said that threat had eased.

"We feel quite confident around the Thomson that we have been able to do everything we needed to stop silt run-off and quality issues in the Thomson,'' she said.

The water is being ordered because water storages are currently at only 57 per cent. ( ABC News: Nicole Asher )

The Government was prepared to use the desal to top-up supplies in the event of contamination.

"If you have a look at the dry conditions and our storage levels [and] the predictions from the bureau I'm pretty confident that advice to me will be we'll need a substantial desal order,'' Ms Neville said.

Last year the state ordered 46 gigalitres from the Wonthaggi plant, which the minister said had helped keep storages above 50 per cent capacity.

"We can't take any risks here around our storage levels. We have got a hot, dry autumn ahead unfortunately [from] what the bureau is telling us, so I think that we will see one of our biggest orders,'' Ms Neville said.

The desalination plant can produce up to 150 gigalitres a year.

Last year the state ordered 46 gigalitres of water, which helped keep storages above 50 per cent capacity. ( ABC News: Nicole Asher )

Impact of fires on water catchments limited

Fires over the weekend in the Armstrong Creek catchment, which feeds the Upper Yarra dam, Melbourne's second-largest reservoir, have also caused some concern, as have spot fires at the northern reaches of the dam.

But Ms Neville said major efforts had been made to limit any impact on the Upper Yarra, where water had been pumped from the Thomson dam.

There are also long-term risks to water supply from bushfires in water catchments, because regrowing bush sucks up more water from the ground, stopping flows into dams.

Dr Chris Taylor from the Australian National University said the fire in the Armstrong Creek area was of particular concern because the area was also burnt by the Black Saturday fires.

"Fires in these forests were historically infrequent, with return intervals spanning decades or even centuries,'' Dr Taylor said.

"The occurrence of this fire so soon after the Black Saturday fires is of great concern, because this is not enough time for the catchment to recover from the previous bushfire."

Some catchment areas can also be logged, further reducing flows into dams.