Melburnians could be drinking water from Victoria's desalination plant – or even be put on water restrictions – if a fire on the banks of a major water catchment east of the city is not quickly brought under control, the Water Minister has warned.

Lightning sparked a bushfire about a week ago in thick forest in the Baw Baw National Park, about three hours’ drive east of Melbourne.

Thomson Reservoir supplies around 60 per cent of Melbourne's water storage capacity. Credit:Vince Caligiuri

Tough terrain has made fighting the fire almost impossible, and the blaze has slowly spread to the edge of the Thomson Reservoir, the largest of Melbourne's water catchments. The Thomson makes up about 60 per cent of Melbourne's total storage capacity.

Ash and runoff from the fire was placing that water supply at risk, Water Minister Lisa Neville told media on Friday, soon after taking a flight over the blaze.