Victorians are being warned a storm forecast to be the biggest of the year could cause flooding across the state on Thursday.
Premier Daniel Andrews said full catchments and saturated ground had elevated the flood risk across the state.
"Even a minor amount of rain would be a risk in terms of flooding, but it's not a minor rain event that we are forecasting," Mr Andrews said.
"It'll be significant rainfall in certain parts of the state and that will pose a flooding risk to communities in lots of different places."
The effects of a third successive year of La Niña are being felt across the country, with tropical moisture building over the top end of Australia.
The Bureau of Meteorology (BOM) said a combination of warm water and air moisture was bringing increased rainfall to the south-east of the country.
BOM meteorologist Kevin Parkyn said the heavy rainfall last week was a "pulse" of the tropical moisture, with the rain this week expected to be more significant.
Mr Parkyn said he was most concerned with the intensity of the rain which was expected to peak on Thursday.
"Wednesday is a day when we'll see showers and storms build across Victoria as the day unfolds," he said.
"The real concern though is from the very early hours of Thursday morning right through to Friday morning where we're expecting widespread falls of 20 to 50mm across much of Victoria.
"I can't stress the importance that Thursday is very much a flash flood, heavy rain day. Be prepared."
SES tells residents to prepare for flooding
Short-term flash flooding is being forecast, while longer term riverine flooding is also expected.
Victoria's State Emergency Service (SES) has asked residents to begin preparing for the storm.
"Now is the time to be clearing out your storm pits, your gutters around your house and making sure you don't have any debris lying around that may become a missile during strong winds," SES chief officer Tim Wiebusch said.
The SES said residents in flood-prone areas should prepare for up to 72 hours of potential isolation.
Mr Wiebusch said residents should be aware that roads would be cut off due to the expected flooding and asked anyone who must travel to plan accordingly.
"This Thursday, we're asking people to avoid driving across particularly the northern parts of the state with these flash floodwaters that will be about," he said.
"Don't drive into flash floodwaters, because it could be the last decision you ever make."
Victorians in the state's north have been told they are most at risk from flooding, with hundreds of thousands of sandbags to be made available at local collection points in high-risk areas.
Residents have not yet been asked to evacuate but updates will be posted on the Vic Emergency app should conditions change.
Seven helicopters and hundreds of generators are on stand-by around Laverton, in Melbourne's west, to help residents deal with flash-flooding where needed.
Mr Andrews also said the government had prepared "community containers", which could provide communities isolated by floodwaters enough supplies to sustain 50 people for five days.
No formal request has been made to the Australian Defence Force for support, but Mr Andrews said he would not hesitate to seek support if necessary.
Meanwhile, regional communities are bracing for more wet weather following last week's downpours.
In Charlton, a town of 1,100 halfway between Melbourne and Mildura, the Avoca River peaked at 6.78 metres on Monday night.
The peak was just shy of the moderate flooding level, after heavy rain upstream made its way down the river.
But local residents told the ABC they feared with more rain forecast, the risk of flooding remained serious.
Authorities said the wet conditions would also affect Tasmania, New South Wales and parts of South Australia.
