Bushfires in Gippsland have burnt 13 per cent of the forest around the Thomson Dam, Melbourne's major water catchment, leaving blankets of ash that threaten to pollute the city's water supply.

Heavy rain, while desperately needed after an extremely hot and dry summer, would only worsen the city's water outlook, as it would wash the fire refuse into the vast reservoir, rendering it unusable for months.

Up to five per cent of the forest around the smaller Upper Yarra reservoir east of Melbourne has also been burnt.

A record 125 gigalitre order has been made from Victoria's desalination plant.

The Andrews government announced on Monday that it had ordered 125 gigalitres of desalinated water from Aquasure, by far the largest purchase made from the company's Wonthaggi plant since it opened in 2012.