Thousands of Australians were evacuated Wednesday, with dozens being rescued, as monsoonal rains sparked flash flooding and landslides and a cyclone threatened to form in the Pacific Ocean.

Emergency authorities ordered 4,200 residents of five towns in New South Wales to abandon their homes as a huge storm lashed northeast Australia, causing rivers to break their banks.

"Do not delay your evacuation. Roads will be congested or closed. You could become trapped and need rescue," the NSW State Emergency Service said.

"Remaining in flooded areas is dangerous and may place your life at risk."

The downpour caused flash floods and landslips in neighbouring Queensland overnight, swamping homes and forcing hundreds of roads to close, with 26 emergency rescues as cars were swept into surging waterways.

Queensland Premier Anna Bligh said the downpour, which has flooded homes was a terrifying reminder of last year's devastating floods which killed 35 people and inundated tens of thousands of homes.

"The deluge has caused significant distress from people who experienced last year's devastating floods, with police fielding numerous calls from anxious victims," Bligh told reporters.

"It's an event that is causing a lot of disruption."

A severe weather warning was lifted for Queensland as conditions eased but Bligh said the rain would return late Friday or Saturday, pelting the state with up to 300 millimetres (12 inches) of rain.

"We will feel the full urging of the monsoons, a trough that has been impacting the north," she said.

A "third phase" of weather was expected later next week, she added, with modelling suggesting a cyclone could form.

"There is a possibility of a cyclone forming off the northern parts of the state ... which will bring with it winds and heavy rainfall," she said.

Top-intensity Cyclone Yasi tore across the Queensland coast last February, ripping homes from their stumps and wiping out huge sections of farmland as the state reeled from an earlier flooding emergency.

The Bureau of Meteorology issued a severe weather warning for northern NSW on Wednesday, forecasting "heavy rain which may lead to flash flooding".

Several local rivers were already breaking their banks, the bureau said, with a "greater than 70 percent chance of major flooding as well as local flash flooding from today onwards."

The exceptional rain, due to a La Nina weather pattern in the Pacific Ocean, is not expected to rival last year's wild storms.

La Nina is characterised by unusually cool ocean temperatures in the eastern equatorial Pacific and has been associated with strong rainfall in Asia and Australia.