A dozen people injured ahead of Vic super storm

A dozen people have been injured while preparing their properties for freak storms expected to dump two months' rain on Victoria in three days.

Eleven men and one women fell from ladders on Thursday, many while clearing their gutters ahead of the deluge.

About 100mm of rain has been forecast for Melbourne between Friday and Sunday, and the worst is expected to hit in the next six to eight hours.

The Bureau of Meteorology said 15 millimetres of rain could fall in just 20 minutes if the thunderstorms hit the state's west.

Nine of the 12 ladder victims were aged over 50, Ambulance Victoria says.

The most seriously injured, a man in his 80s, was airlifted to Melbourne with critical head and spinal injuries after falling near Bendigo.

Flooding is predicted in several areas across the state. Source: 7 News

The others sustained head and back injuries, bruises and fractures.

"This is an immediate consequence of someone doing the right thing, and the safety issues are being compromised," Victoria's Emergency Management Commissioner Craig Lapsley told ABC radio on Friday morning.

The Bureau of Meteorology earlier warned lives were at risk because of the deluge, the biggest set to hit Melbourne in over a decade.

"This is an event of absolute massive scale, half the inhabitants of Melbourne have never ever seen anything like this " senior meteorologist Scott Williams said on Thursday.

"It is an event that poses a threat to life. There will be a massive amount of lightning, there will be roads cut and flood waters.

"This event will turn farms into lakes with such rapid rain rates."

A thunder storm passes Port Campbell as seen from Gibson Steps in Western Victoria, on Thursday. Photo: AAP

When asked how bad the rain event would be on a scale of one to 10, Mr Williams said he'll "take a punt and say it's a 10 for Victoria".

Unharvested crops are at risk of being wiped out, with farmers in some parts of the state working around the clock since Wednesday to finish harvesting.

People in both metro and regional areas have been warned not to become complacent if the rains haven't hit hard by Friday morning.

"They didn't think the Titanic would sink, but it did two hours later," Mr Williams said.

Authorities have also urged people to stay home and avoid travel where possible.

It only takes 15cm of water to float a car, SES deputy chief Tim Wiebusch warned.

Severe rain is set to hit Melbourne around lunchtime. Source: BOM