People in low lying areas around Myrtleford in Victoria's alpine region should prepare to evacuate as floodwaters caused by unprecedented rain continue to rise.
The evacuation warning, issued on Friday night, said people should leave by 1.30am.
"If you do not evacuate before this time, your property may be isolated or inundated and it may be too late to leave," the warning says.
A relief centre has been set up at the Myrtleford Senior Citizens Centre, the safest evacuation route along the Great Alpine Road.
There are major flood warnings in place for the Ovens, King, Buckland and Buffalo rivers and a moderate warning for the Murray River upstream of Lake Hume and Kiewa River.
Melbourne is expected to see a month’s worth of rain in just two days as Victoria braces for the worst storm in almost a decade.
Emergency Management Commissioner Craig Lapsley said the metropolitan area had already seen 30mm of rainfall on Friday.
"Now that is about half of what the average December rainfall would be. So on the first day of December we've already got half of what the average for the month would be," he said.
"And we expected overnight to receive another 30, 40mm of rainfall in the metropolitan area, which would see the average December rainfall meet that in the first two days of December."
Mr Lapsley urged people to remain vigilant even though the rain was not consistently falling hour-by-hour, as he said it had been "very isolated and very direct and significant".
"If we were to receive the 30, 40 mm in the night hours of Melbourne, think about it, you go to bed and you wake up your backyard is full of water and the streets look different," he said.
"That's why the decision tonight about where you go and what you do is very important."
Mr Lapsley said there was a watch and act in place for people in the Elwood-St Kilda area as the Elwood Canal was close to breaching - with properties under threat if the rain continues into the night.
The main areas that would be impacted on Saturday, which was forecast to have the most rainfall, was Victoria's north-east, with up to 250mm of rainfall expected, the commissioner said.
"So the key thing is about information and decision-making, make the decision early about what you need to do for you, your family, your friends, your neighbours and those ones you love, and some people will need a hand," Mr Lapsley said.
The Hurstbridge and Melbourne/Albury train lines closed.
Mr Lapsley said the worst was "absolutely" yet to come.
"We're not out of it. It hasn't necessarily been across the state yet but it's coming and the bureau has been very clear in their forecast," Mr Lapsley said.
"In some cases rain has been very isolated, very direct, significant downpours."
Homeless people sleeping in Melbourne's CBD will be offered free accommodation.
12 injured while preparing for downpour
A dozen people have been injured while preparing their properties for freak storms expected to dump at least two months' rain on Victoria in three days.
Eleven men and one woman fell from ladders on Thursday, many while clearing their gutters ahead of the deluge.
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.
The others sustained head and back injuries, bruises and fractures.
In just 30 minutes, 20mm of rain fell over Ferny Creek, in Melbourne's south-east, and 25mm in an hour in Doncaster, in the city's north-east, according to Weatherzone.
Some farmers have been working around the clock since Wednesday, frantically harvesting their crops before the deluge arrives.
Qantas flight QF94 from Los Angeles to Melbourne has been diverted to Sydney with more international diversions expected.
Meanwhile, a number of domestic flights have been delayed and some cancellations, with Melbourne Airport urging passengers to check for changes.
'Massive scale weather event'
Prime Minister Malcolm Turnbull warned southern Australians to take care during the wild weather.
"Take care in southern Australia during the extreme weather and look out for family and neighbours. Be prepared, and follow advice from the emergency authorities very carefully. Don't drive into flooded waters - if it's flooded, forget it," he tweeted.
The Bureau of Meteorology on Thursday issued a severe rain warning for the entire state with predicted flooding in all of the state's rivers, including the Yarra in Melbourne.
The warning was supported in parliament on Thursday by Victorian Premier Daniel Andrews, who urged the public to stay safe and heed advice from authorities in the coming days.
"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.
"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."
Mr Williams said on a scale of one to 10, he'll "take a punt and say it's a 10 for Victoria".
Worst rain event in a decade
A rain event of this forecasted magnitude has not been seen in metropolitan Melbourne since 2005 and in regional Victoria since 2010, SES deputy chief Tim Wiebusch told reporters.
It only takes 15cm of water for a car to float and people should not attempt to drive through floodwaters, he said.
Emergency Management Commissioner Craig Lapsley said people should stay home and avoid driving on the state's roads.
People are being warned not to become complacent if the rains haven't hit by Friday morning, with Mr Williams adding: "They didn't think the Titanic would sink, but it did two hours later."
SES on standby
South Australia's emergency control centre has been activated ahead of severe thunderstorms set to dump 100mm of rain over the state in coming days.
The SES has staff rostered 24 hours through to Monday with all units across the state on active standby.
Duty Officer Sara Pulford said the SES expected to be busy.
"Up until 7pm tonight, the SES received 39 calls for assistance from the South Australian public," Ms Pulford said on Thursday night.
"We expect this number to increase rapidly as the front moves through the Adelaide metropolitan area and the Mount Lofty Ranges later tonight."
Ms Pulford said the SES had already been called to fallen trees and dropped branches and warned people to remain vigilant around large trees.
Sandbags were distributed to four locations in the Adelaide metropolitan area and Mount Lofty Ranges and Port Pirie Council also has sandbags available from two locations in Port Pirie and Crystal Brook.
The bureau warns winds gusts of up to 90 km/h are expected and a flood watch has been issued for the Torrens, Onkaparinga and Gawler rivers.
Bushfire threat in the east
An out-of-control bushfire continues to rage in Victoria's east, with an evacuation warning issued for nearby residents.
Amid torrential rain across the state, a 4000-hectare blaze is burning near Cann River in East Gippsland, travelling southeast towards Point Hicks and Wingan Inlet.
The warning is in place for Point Hicks, Furnell, Tamboon and campsites at Peachtree Creek and Choof Choof.
People have been urged to leave if it is safe to do so as conditions may change and worsen quickly.
"Emergency services may not be able to help you if you decide to stay," the warning issued on Friday afternoon said.
More than 100 firefighters from the Department of Environment, Land, Water and Planning are battling the blaze.
There are road closures at West Wingan Road, East Wingan Road, Stony Peak Road, Tamboon Road, Thurra Road and Point Hicks Road at Fishermans Track.
- with additional reporting from Louise Cheer
