Heavy rain is expected to drench southeastern Australia over the next few days and cause flooding in parts of Victoria and NSW.
The Bureau of Meteorology says a significant thunderstorm and rainfall system will move over South Australia on Thursday before hitting Victoria, southern NSW and parts of Tasmania in the next two days.
Southeast Queensland copped a drenching on Wednesday with 111mm recorded between 9am and 3pm at Mt Coot-tha in Brisbane.
Forecast storms have the potential to cause widespread flooding in parts of Victoria and southern and central NSW, the bureau said.
Parts of Victoria are set to receive up to three times their monthly rainfall in as many days.
Flash flooding is likely in Melbourne as a low-pressure system dumps up to 150mm of rain on the city between Friday and Sunday.
Falls in Victoria's northeast could top 300mm.
Elderly lady piggy-backed to safety from Brisbane’s rising floodwaters
Tree branch crashing onto Sydney woman's car - so who's liable for damages?
"This is a very, very big weather event, we are in uncharted territory," senior meteorologist Scott Williams told reporters on Wednesday.
"We could see six-hourly rain rates of over 100mm over several parts of the state, stretching from the Grampians and Horsham in the west to the northeast and ultimately Melbourne."
It follows an unusually hot and humid November, with Melbourne breaking its record for the number of straight days over 30C.
Victorian farmers have worked frantically through the night, harvesting as much produce as possible before the rain hits.
Mallee farmer Nick McClellan worked until 4.30am to harvest his crop of barley, chickpeas and legumes.
"The rain will probably beat us ... we've probably got three or four days to go and the rain will be in tomorrow," he told 3AW radio.
Severe thunderstorms are also forecast for Victoria's southwest on Thursday.
On Wednesday, an elderly Queensland driver was piggybacked to safety after she became stuck in flash flooding in Brisbane.
Bureau of Meteorology forecaster Lauren Murphy said the rain had been widespread across Brisbane.
The average rainfall was between 60 and 70mm between 9am and 3pm on Wednesday.
Mt Coot-tha recorded 111mm while around 90mm fell on Alderley and Bardon.
Ms Murphy said showers were expected to continue into Thursday.
Today's top videos