Dangerous, strong winds gusting up to 90km/h are also forecast for coastal areas as a low-pressure system builds over the ocean near the Illawarra region.
However one of the biggest concerns for forecasters and hydrologists is the Warragamba Dam, which is at about 98 per cent capacity.
The dam has spilled several times in the past, most recently in 2013 and again in August 2015.
"It's possible that the dam may spill, but it depends on how much more rain it gets and how widespread across the catchment that rainfall is," a Bureau of Meteorology forecaster said.
Flood watches have also been issued for several river catchment areas from the Tweed to the Illawarra but the BoM is waiting for further rainfall data before issuing warnings.
Over the past two days Water NSW has warned heavy rainfall could cause the dam to spill.
It is the second bout of wild weather to lash the east coast in two weeks, leaving five people dead and a multi-million dollar clean-up bill.
During the last deluge, more than 300 flood rescues were carried out - and more than 30,000 insurance claims worth a combined A$235 million across east coast states and Tasmania were lodged in the aftermath.
Bureau of Meteorology forecaster Gabrielle Woodhouse said while the worst of the rain had left NSW, there were still severe weather warnings in place.
More than 70mm of rain fell in Brisbane in an hour yesterday, leaving thousands of homes without power while flash flooding struck several roads.
A woman and her daughter were rescued from their stranded car in the southern Brisbane suburb of Coopers Plains.
"The water come really quick. When we just got here the water is not too much, it wasn't too high, but because everywhere (was) blocked we just tried to go in and then we got stuck there," the woman told the Nine Network.
Several streets in Brisbane were flooded after 106mm of rain fell on the city throughout yesterday - more than half the total coming in the evening.
The South Australian desert has been transformed into a lush oasis after flooding rains around Lake Eyre.
Marree Hotel manager Rebecca Turner says she's waiting for the wildflowers to bloom after months of wet weather. "It doesn't reflect what the desert actually looks like, it's too green," she said. "Parts of the Birdsville and Oodnadatta tracks are looking absolutely stunning."
- news.com.au, AAP
