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Extreme rain is rolling into Victoria which could threaten lives, flood rivers, turn farms into lakes and cause major flash flooding in Melbourne.
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".
A rain event of this forecasted magnitude hasn't 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."
The low-pressure system may dump more than 300mm of rain in Victoria's northeast and up to 200mm over Melbourne.
That's the equivalent of two to three times the monthly average in two days.
Some farmers have been working around the clock since Wednesday, frantically harvesting their crops before the deluge arrives.
City dwellers were also being put on high alert for flash flooding, which was likely to occur too quickly for warnings to be put out, Emergency Management Victoria said.
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.
A low-pressure trough is expected to bring thunderstorms across the state on Friday, after heavy rain began falling on Thursday, and residents are being warned to prepare for flooding.
Bureau of Meteorology senior forecaster Matt Collopy says some parts of SA could be hit with record rainfall for this time of year.
"That 50mm to 70mm of rainfall in some cases can represent two months worth of summer rain," he said.
The SES has staff rostered 24 hours through to Monday with all units across the state on active stand-by.
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.