Melbourne could see as much as two months' rain in the next three days.
Heavy rain and scattered thunderstorms will develop across the state and hang around until Sunday with flash flooding likely and a severe weather warning in place for the entire state.
The worst of it is expected to hit Melbourne late Friday or early Saturday.
Forecaster Chris Godfred telling Triple M's Hot Breakfast records could be broken over the next few days.
"Melbourne could be getting totals around about 60-100mm," he said. "Basically about two months worth of rain falling in the space of about three days."
While the Bureau of Meteorology's Scott Williams expects some of the worst conditions in decades.
"This is a very big event," said Mr. Williams. "It's right at the top end of the rainfalls we've seen in Victoria in the last 30 years."
A large high pressure system near New Zealand directing a hot and humid northeasterly airstream over Victoria with a trough of low pressure crossing the Bight to hit western Victoria late on Thursday.
Totals over the northeast ranges may exceed 250mm.
Warnings are in place for Mildura, Horsham, Warrnambool, Bendigo, Shepparton, Seymour, Maryborough, Ballarat, Geelong, Melbourne, Wodonga, Wangaratta, Traralgon and Bairnsdale.