Authorities moved to evacuate thousands of people from a second Australian town today as floodwaters from days of torrential rain inundated large parts of the country's east coast.

About 5,000 residents in Lismore, in northern New South Wales (NSW) state, were evacuated from their homes as floodwaters, in some places more than 10 metres deep, surged across riverlands stretching along 300 km of coastline.

Authorities also urged 9,000 people to evacuate the nearby town of Grafton, with floodwaters up to eight metres deep expected to hit the town late today. State authorities declared disaster zones in six areas, unlocking government help.

"The declaration will provide for a range of assistance to cover personal hardship and distress as well as funding for those who have suffered property damage," NSW Premier Nathan Rees said.

The flooding followed days of rain and cyclonic winds which battered southern Queensland state and northern NSW, killing one man and trapping hundreds of people as roads were cut, forcing authorities to use helicopters to rescue some.

The storms left 16,000 people without power as fallen trees brought down lines and blocked roads.

Floodwaters washed cars into the sea and huge waves pounded major ports, including the world's biggest coal export port at Newcastle, where ship movements were disrupted.

"There have been some vessel disruptions, but effects on coal throughput are negligible," a spokesman for Port Waratah Coal Services Limited told Reuters.

Rivers burst banks at the towns of Murwillumbah, Tweed Heads and Grafton, although major coal-mining operations to the north and west in Queensland were unaffected.

Prime Minister Kevin Rudd offered help to flood-hit areas.

"It is critical residents follow the instructions of local authorities and as far as possible stay indoors and avoid the roads," Rudd said.

Further south near Coffs Harbour, the Bellinger River was expected to peak at more than eight metres, with more than 3,000 people isolated in the towns of Thora, Darkwood and Bellingen.

In southeast Queensland, hundreds of roads remained cut, with homes flooded and industrial suburbs in popular tourist beach areas transformed into canals.

While the rain had mostly moved south, a massive clean-up operation in the state continued as beaches were swamped with huge waves and high tides.