A month's worth of rain is set to fall in Melbourne and other parts of Victoria by Wednesday, with a wider severe weather warning issued for the state.
Thunderstorms swept over Melbourne's north and areas around Sunbury and Whittlesea on Monday night.
In Strath Creek, north of Melbourne, 54 millimetres of rain fell in just 30 minutes around 7pm.
Widespread rain and isolated thunderstorms are expected to develop across most of the state overnight, as a low-pressure trough over South Australia moves across northern Victoria.
"Due to the humid, unstable atmosphere the storms tend to weaken and then redevelop. It's difficult to pinpoint which suburbs will be hit and at which time," Bureau of Meteorology forecaster Peter Newham said.
Some parts of the state could receive up to 60 millimetres of rain in 24 hours.
Areas worst hit may include Horsham, Warrnambool, Bendigo, Shepparton, Seymour, Maryborough, Ballarat, Geelong, Melbourne, Wodonga, Wangaratta and Traralgon.
A severe weather warning for heavy rain and flash flooding was issued for the state at 4pm on Monday.
Melbourne could receive up to 50 millimetres of rain by Wednesday morning - the average rainfall for the whole of March.
The inland trough is dragging moisture down from the north and bringing Queensland's sub-tropical weather to Victoria.
The trough dumped 10 millimetres across Melbourne on Monday morning.
But the downpour is set to come on Tuesday morning, when up to 30 millimetres of rain is expected in the city.
"Keep your brolly in your bag for tomorrow morning," Bureau of Meteorology forecaster Stephen King said.
A few extra showers on Wednesday morning could take Melbourne's rainfall over 72 hours to 50 millimetres.
With the rain comes humidity, and Melburnians can expect another muggy night, with the mercury not expected to dip below 20 degrees.
"It will be hot and sticky until at least Wednesday," Mr King said.
The humidity reached 90 per cent in some parts of the city by lunchtime on Monday.
A top of 25 degrees is forecast for Tuesday, and 21 degrees for Wednesday.
The mercury will rise again at the end of the week, with a maximum of 26 degrees forecast for Friday and Saturday, and 27 on Sunday.
"Hopefully, next week we'll see a return to autumn weather, with crisp mornings, mild days," Mr King said.
In northern NSW, more than 4000 people remained isolated on Monday because of heavy rain, with wet weather forecast for much of the state during the week.
Eleven rivers in the Northern Rivers and Mid North Coast regions flooded. SES crews are continuing the clean-up and urging residents to stay safe.
Significant rainfall eased on Monday, but the SES said about 4200 residents in the Clarence and Nambucca regions remained isolated near Iluka.