Premier Daniel Andrews warns Victorians to prepare for major rain, flooding

"It's not a minor rain event that we are forecasting," he said.

Authorities are deeply concerned about the risk of flash flooding by Thursday along with slower inundations along rivers like the Avoca, Campaspe and Loddon Rivers.

The Bureau of Meteorology expects the rains to hit from Wednesday.

"We have a significant rain event on Victoria's doorstep," BOM meteorologist Kevin Parkin said.

"What I'm most concerned about is the intensity of the rain that will peak on the Thursday.

"So we've got a short-term flash flood risk as a result of the rainfall intensity, and then a longer term riverine flooding risk because it takes a while for some of the waters to find their way down Victoria's river systems."

You need to be prepared, authorities are saying.

That includes readying for as much as 72 hours of isolation, Emergency Victoria deputy commissioner Chris Stephenson said.

"And that includes making sure it's not just yourself but your neighbours," he said.

People will need to provide for their pets and make sure they have their own medications handy, "because we need our first responders to actually respond to those vulnerable people - those people that need them most", Mr Stephenson said.

The biggest concern will be deluges excepted to hit from early Thursday morning through to Friday, at this stage.

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Some catchments north of the Great Dividing Range could cop between 60 and 100mm.

A host of those areas already have minor flooding and could be inundated.

Emergency services are prepared for a major response across the state, Mr Andrews said.

"There are a couple of hundred generators out of Laverton waiting to be deployed to communities who may lose power," he said.

"There are seven choppers that are on standby and more that can be accessed if we need those. They'll be forward deployed as well once we have a clearer picture of where the most significant rain event has, in fact, unfolded."

Emergency crews are preparing hundreds of thousands of sandbags for rollout, Mr Andrews said.

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