Victorians will bare the brunt of a severe tropical cyclone and a low-pressure system with a month's worth of rain, thunderstorms and hail expected to hit.
The wrath of tropical Cyclone Owen will be dragged into a low-pressure system forming over Victoria on Thursday, bringing rain, hail and thunderstorms.
"We have a couple of major systems over the coming few days that will impact large parts of eastern Australia," Bureau of Meteorology extreme weather desk manager James Taylor said on Wednesday.
"It will create a heavy rainfall risk from a cloud band with embedded thunderstorms over a large part of probably the eastern seaboard.
"There is also damaging to destructive winds associated with tropical Cyclone Owen that we need to worry about."
A low-pressure system is expected to develop over Victoria on Thursday and draw in moisture from the tropics and the cyclone, he added.
It is expected to dump between 50 to 100mm of rain in Victoria's northeast and central areas, including Melbourne, depending on thunderstorm activity.
Otherwise "a good chunk of Victoria" is forecast to have between 30 to 50mm on Thursday.
It is due to bring the average December rainfall in one hit, Mr Taylor added.
"We are looking at a very wet day across a large part of a Victoria," he said, before it begins to clear on Friday.
"A month's worth of rainfall is a really good way of describing it."
Hail stones about 2cm in size could fall as damaging winds with gusts up to 90km/h sweep through the warning areas.
Mr Taylor could not rule out flash flooding and rainfall similar to that experienced in Sydney in recent weeks.
"Sydney had up to 70mm in an hour. Could we see rainfall rates like that with this? It is not out of the question," he said.
Tropical Cyclone Owen has been upgraded to a category two as it moves west in the Gulf of Carpentaria, and is due to be raised to a category three in the morning.
There is a rainfall risk over large parts of Victoria, northern and eastern Tasmania with scope to move into South Australia, eastern NSW and along the Queensland coast.
Mr Taylor couldn't say if the rain would break the drought in NSW and Queensland, as it would be "quite hit and miss".
Australian Associated Press