Much of South Australia and Victoria and parts of NSW and Tasmania are bracing for flooding over the next few days as thunderstorms hit the nation's southeast.
The wet start to summer is being driven by a significant thunderstorm and rainfall system forecast to move over SA on Thursday, before hitting Victoria, southern NSW and parts of Tasmania in the next two days.
The Bureau of Meteorology believes the storms have the potential to cause flooding across large parts of SA and Victoria and southern and central NSW.
In NSW, it expects flooding could be most significant along the Tumut and Central Murrumbidgee rivers in the Murray-Darling basin.
Flooding could also hit Tasmania, the bureau has warned, along the North Esk and South Esk River Basins - in the state's north - and the Huon River, in the south.
Senior meteorologist Adam Morgan said the heaviest downfalls were expected between Friday and Sunday over northern Victoria and southern and central NSW.
"This is a significant rainfall event for eastern Australia," Mr Morgan said.
"Many locations will see well over their December average of rainfall in just the first couple of days of the month.
"This amount of intense rainfall is going to lead to both riverine flooding and flash flooding in some areas."
Initially, the storms are expected to bring damaging winds, hail and heavy rainfall of up to 100mm across swathes of land lying to Adelaide's north and the Nullarbor late on Thursday and into Friday.
The storms are then expected to move east into Victoria and western NSW on Friday and Saturday, with rainfalls up to 200mm.
Northeastern parts of Victoria are expected to feel the brunt of the storms, with more than 250mm of rain expected between Thursday and Saturday.
Rainfall of at least 50mm to 100mm has been forecast for Melbourne while southern parts of NSW and the ACT could see falls in excess of 100mm.
While the heavy rain could reach Canberra, Sydney is likely to miss out.