Some parts of the country are expecting to see several times the average November rainfall in just the next few days.
It come as an intense low pressure system moves across much of the interior and east coast of Australia.
"Eastern Australia is set to be hit by widespread storms, intense rain and potentially major flooding as a weather system impacts millions of people," said Jonathon How from the Bureau of Meterology.
A tropical air mass has moved over Western Australia, South Australia and the Northern Territory, on its way towards the east coast.
It is rare to see this kind of tropical weather event this early into the storm season.
By Wednesday evening, storms will cover over half the continent.
It's bringing with it humid and unstable air that is likely to promote large storms along the east coast, with between 150 to 200mm of rainfall expected in some areas.
In Victoria and Tasmania, the storms will also bring a significant cold front.
"Rainfall totals this week could be record breaking," said Mr How.
"By week's end, some places could have seen the November rainfall average several times over."
The worst of the weather is expected on Friday, when damaging winds will likely be seen around Victoria and NSW. Some conditions could continue into next week.
Some flooding is expected around key catchment areas, including along the Northern and Central Tablelands, North West Slops, Riverina and Mid North Coast.
Flood warnings remain current for the Lachlan River, Macintyre and Weire Rivers. But flood risks have been downgraded for the Wilsons River, Richmond River, Madagery Creek, Murrumbidgee River and Burrinjuck Dam.
However, flash flooding may still occur across the nation as a result of the high rainfall.
"This rain is expected to be very heavy at times," said Jane Golding, Bureau of Meteorology Hazard Preparedness and Response East manager.
"The soils are saturated across NSW at the moment. We're expecting it to lead to some riverine flooding but also to some dangerous flash flooding and road flooding."