A number of dams and reservoirs are threatening to overflow or even burst as heavy rain is dumped across Victoria.
Heavy rain and thunderstorms are continuing and some property owners in Victoria's north-west have recorded more than 90 millimetres of rain over the past two days.
Cars have become stranded in flash flooding in South Melbourne and Frankston where a Watch and Act warning was issued for low-lying areas.
Emergency crews rushed to Craigieburn in Melbourne's north as a private dam threatened to collapse and flood nearby homes.
Mickleham Road was closed while State Emergency Service crews assessed the situation.
Trees have been brought down and buildings damaged in Heidelberg, in Melbourne's north-east.
Watch and Act warnings have also been issued for flash flooding in Elwood in bayside Melbourne, and for riverine flooding in central and north-western Victoria with moderate flooding occurring along the Murray River at Corowa, Laanecoorie Reservoir, south-west of Bendigo, and predictions of flooding in Charlton by late Saturday.
The Bureau of Meteorology's Matthew Thomas said the rain would ease and there would only be isolated showers over the weekend.
But he said heavy downpours would return for much of the state on Wednesday.
"We're looking at 5 to 20 millimetres into the west and the north and central parts of the state but 20 to 40 millimetres about the north-east ranges," he said.
"But that's really a build up to some tropical moisture that will then feed ahead of a cold front across next Thursday and it's looking quite wet about central and eastern parts of Victoria."
He said some areas could receive up to 70 millimetres.
Rod McErvale is a farmer at Lexton in central Victora.
He said heavy rain fell from the early evening and within an hour the area around the local creek was in flood, with waters reaching his home.
"At midnight, I've never seen it that high," he said.
He said he did not expect the rain to be so heavy and he had lost a lot of fencing.
"Six to 7 inches higher than the 2011 flood and a lot stronger so it's done a lot more damage," he said.
He said communities affected by the 2011 floods would also be inundated today.
Fears of repeat of 2016 thunderstorm asthma
It comes as Melbourne residents are being warned to prepare for a thunderstorm asthma event similar to the 2016 tragedy.
High grass pollen counts are expected this spring due to the wet winter and the flooding rain this week.
Thunderstorm asthma happens when gusty storms from the north-west coincide with high pollen counts, and can particularly affect people who suffer from hayfever.
Melbourne experienced the worst thunderstorm asthma event in 2016, when 10 people died and thousands suffered breathing difficulties after a severe storm swept into the city from the west.
Respiratory medicine specialist and director of research at the Royal Melbourne Hospital, Jo Douglass, said grass growth across the state had created the increased risk.
"If we get the storms and hot weather with high pollen counts, then we'll certainly be at high risk," Professor Douglass said.
"The particular threat is often after a few warmer days when the pollen counts get really high then a real storm from the north-west that can really drive the thunderstorm asthma conditions.
"We think the high pollen counts render that likely. We obviously can't predict it perfectly but it is likely to be quite a dangerous year."
Professor Douglass described it as an "environmental health emergency".
She said for many people, it escalated hayfever to asthma.
"Whilst two-thirds of people probably knew or had symptoms of asthma before the 2016 event, a third didn't know they had asthma but nearly all of them had hayfever," she said.
"So hayfever is certainly a risk for thunderstorm asthma and then if you do know you have asthma, you're definitely at risk."
Professor Douglass said Melbourne had good warning systems and the Emergency Victoria mobile phone app now issued warnings for high pollen counts.
She said the risk would remain until the end of December and people should use COVID face masks to protect themselves.
"Stay out of the storm front, stay inside, close the windows … but then an N-95 would also be helpful," she said.
She urged hayfever sufferers to take their medication and people with asthma to take their preventers and always carry their Ventolin.
Emergency response to unprecedented tragedy
While 2016 was not Melbourne's first thunderstorm asthma event, the health department said it was unprecedented in its size, severity and impact.
On November 21, in the 30 hours from when a gusty storm hit Melbourne at 6pm, the department said there was a 672 per cent increase in respiratory-related presentations to Melbourne and Geelong public hospitals, making it the largest incident of its type ever recorded in the world.
The department said since then, a comprehensive Thunderstorm Asthma Program had been developed which included an epidemic thunderstorm asthma forecasting system and a public warning system.
Hospital and GP guidelines have been updated to identify and manage those at increased risk.
Environment Minister Lily D'Ambrosio said people who had hayfever or asthma must see their GPs to make a plan.
She urged Victorians to take the warnings seriously.
"These things of course can turn around really quickly and can have significant impacts really quickly," she said.
An inquest into the 10 deaths in 2016 heard the event sparked the "single greatest volume of calls for ambulance assistance" in Victorian history but the triple-0 call service, ESTA, was not prepared for the unprecedented surge and some callers were told an ambulance was on its way when none had been dispatched.
The coroner recommended callers be given an estimated time of arrival for ambulances to allow them to consider organising their own transportation.
Ms D'Ambrosio said improvements had been made to ESTA funding and operations.
"People can be very, very confident of the ability of our emergency services to respond," she said.
