Flood warnings are in place for New South Wales and Queensland after heavy rainfall overnight.
It comes as the two states received more than a month's worth of rain in a day.
The deluge has caused Sydney's Warragamba Dam to hit capacity and begin to spill.
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The dam began spilling just after 9pm last night.
The spill will peak mid-morning at about 100 gigalitres per day and is expected to continue for a week or more.
The weather has turned deadly after a man was killed in central Queensland.
The man's ute was swept away by floodwaters in Hibernia, west of Rockhampton in central Queensland.
A woman is also fighting for life in Sydney after a tree fell on a car at Glen Alpine in the city's south-west.
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The heavy rainfall and wild weather has caused damage to NSW roads.
A bridge collapsed in the NSW Central West as a result of the deluge.
A car was driving over the Nyrang Creek Bridge near Canowindra as it was collapsing.
The occupants were taken to hospital and were not in a serious condition.
NSW Premier Dominic Perrottet has urged residents to pay attention to weather and flood warnings.
"We've had over 34 flood rescues in the last 24 hours that is way too many," he said.
"Please don't drive through floodwaters, please follow the advice you've been given. That will keep you and your family safe."
The NSW State Emergency Service (SES) received more than 850 calls for help overnight and performed 34 flood rescues.
There are flood warnings in place for NSW inland rivers, and the Hunter, Hawkesbury Nepean Rivers.
Minor flooding is expected for the Hawkesbury Nepean River this afternoon.
It comes just months after the region was hit by devastating floods which destroyed homes and businesses.
Scone in the state's Upper Hunter region has been hit particularly hard with residents told to prepare for evacuations.
In Queensland, there are flood warnings in place for the Dawson River and Theresa Creek.
Australia's east coast has been battered by a weather system that stretches across four states as the November deluge continues.
Bureau of Meteorology (BoM) senior meteorologist Jonathan Howe said yesterday the rain had been "relentless" across Queensland and NSW.
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NSW
More "unsettled" weather is expected in NSW through the weekend as a trough moves east, the BoM said.
Showers will continue over the east and north today with the potential of a severe thunderstorm in the north-east.
Temperatures will be below average today.
Queensland
The deluge is showing no sign of slowing with severe thunderstorms, damaging winds and rain forecast across the state today.
A trough over central and southern parts of the state will move east today.
There is a chance of showers and thunderstorms on the east coast and far north.
South Australia
The state will remain mostly dry with a chance of rain near the Eyre Peninsula.
Moderate winds are expected.
Western Australia
Showers and thunderstorms are forecast over the north Kimberley.
Tasmania
It will be mostly sunny with light winds.
Northern Territory
It is forecast to be partly cloudy with a high chance of rain and thunderstorms.
Australian Capital Territory
The BoM reports it will be cloudy with a chance of showers.
