Widespread flooding in Australia left hundreds needing to be rescued after a natural disaster was declared in regions across New South Wales on Sunday.

Intense rainfall at the start of the weekend saw around 300 homes evacuated in north-east Sydney, emergency services said.

Floodwaters had caused widespread damage across the state in a very short amount of time, federal minister for emergency management Catherine King said.

"The New South Wales government is continuing to assess the damage and understand the impact of this disaster," King added.

Emergency crews had performed about 200 flood rescues since Friday, said state minister for emergency services Jihad Dib, as some regions were left cut off by damaged or flooded roads.

Around 5,000 volunteers had worked through the night helping those affected, he added.

State emergency commissioner Carlene York said 60 community warnings remained in place for regions affected by the flooding.

Disaster support had been triggered by officials in a dozen areas to help cover clean-up costs and pay for emergency accommodation. It was the region's seventh flood in 18 months.

Researchers have repeatedly warned that climate change amplifies the risk of natural disasters such as floods, bushfires and cyclones.