Four dead in devastating Aust floods

Four people are dead and thousands of homes are underwater in Queensland and NSW as ex-tropical cyclone Oswald closes in on Sydney.

The weather front is beating a devastating path down the coast and authorities are ramping up for another day of evacuations while floodwaters have already isolated hundreds of homes.

The crisis has claimed four lives so far, including a three-year-old boy who was hit by a falling tree in Brisbane's north.

Eighteen people have been rescued with the most recent operation taking place overnight at Lismore, where a man was plucked from the roof of his car.

State Emergency Services (SES) Deputy Commissioner Steve Pearce said some areas in northern NSW had been hit with over half a metre of rain, with more than 2000 people isolated by floodwaters.

After hitting the Hunter on Monday evening, the front was expected to hit Sydney around dawn on Tuesday, he said.

"We're expecting really damaging winds of up 100km/h; we're expecting torrential rain of up to 100mm and possibly 200mm in localised areas," Mr Pearce said.

"We're expecting flash flooding, we're expecting trees to be brought down, wires to be brought down by these winds.

"We're expecting a very challenging 24 hours in front of us."

Shortly before 7pm (AEDT) on Monday, about 1500 residents downstream of Grafton were ordered to leave their homes in Ulmarra, Cowper and Brushgrove districts.

Floodwaters have also cut off all roads between NSW and Queensland.

Meanwhile, more than 2000 homes are underwater in the worst-affected city - Bundaberg in central Queensland.

About 3000 people have also been evacuated two years after floods devastated much of the same areas of the state, resulting in 35 deaths.

Army choppers equipped with night vision were used to evacuate stranded residents into the evening.

"The water is everywhere ... it's the worst on record," the city's mayor, Mal Forman, told ABC TV on Monday night.

Brian Cox, from Queensland's Disaster Management service, said the greatest challenge was ever-rising floodwaters.

He said 190 patients would probably be flown out of Bundaberg Hospital on Tuesday morning.