Torrential rain pummelled Australia's southeast on Thursday, triggering flash flooding and forcing officials to issue fresh evacuation orders, while 50,000 residents were warned to prepare to isolate with more downpours expected over the next 24 hours.

Major flooding is occurring in several rural towns in the Hunter and Mid North Coast regions of New South Wales, Australia's most populous state, with severe weather warnings stretching over a wide swathe of both areas.

Police said the body of a man was found in a flooded home near Taree, more than 300 km north of Sydney. The rural town is one of the worst-hit by the floods, that have washed away farms and destroyed homes, roads and bridges.

"I'm very sorry for that man's passing. We should be bracing for more tough news over the next 24 hours," New South Wales Premier Chris Minns told ABC News.

More than 100 schools are closed on Thursday, while thousands of properties remain without power, authorities said.

A slow-moving coastal trough has dumped about four months of rain over the past two days, cutting off entire towns and stranding residents on roofs and the second storeys of their homes, as rescuers struggled to access the area by boat or air.

Minns apologised to people who had to wait for several hours for rescue crews, but assured efforts had been ramped up with 2,500 emergency services personnel deployed on site.

Television video showed a woman winched to a helicopter from a flooded property.

Australia's Bureau of Meteorology forecast that some areas could receive up to 300 mm of rain through Friday, triggering life-threatening flash flooding, before the weather system is expected to weaken and track south toward Sydney. (Reuters)