SOUTHERN NSW is being told to prepare for rain and 100km winds as ex-tropical cyclone Oswald sparks evacuations in the state's north.

As the impact of the massive storm spreads from Queensland, more than 1500 people have been told to evacuate their homes in Lismore, Ulmarra, Cowper and Brushgrove in northern NSW with warnings of flooding as rivers peak throughout today.

The NSW State Emergency Service says the focus will be on the south of the state today, with a severe weather warning issued for Sydney, the Hunter and Central Coast, Illawarra and the south coast and parts of the Central Tablelands.

In Queensland, four people have been killed by the floods, including a three-year-old boy who was hit by a falling tree in Brisbane's north yesterday, while thousands around the state have been forced to evacuate their homes.

More than 2000 homes are underwater in the worst-affected city, Bundaberg, where the Burnett River is expected to hit its peak tonight.

The Brisbane River is due to peak at 2.6 metres at about 11am (AEST).

Around 180,000 homes are without power in southeast Queensland this morning.

Meanwhile in Grafton in northern NSW, parts of the town was issued an evacuation warning overnight with the Clarence River expected to cause major flooding when it peaks near eight metres at about 9am (AEDT) today.

"The Clarence River flood peak at Lilydale was 20.94 metres at midnight. This is close to the 1954 flood peak of 21 metres," the Bureau of Meteorology said.

Reports that 20 have been stranded by floodwaters in Grafton is also being investigated, NSW SES spokesman Michael Eburn said.

It comes as more than 2000 people remain isolated, with around 850 people isolated at Ewingar west of Lismore and 400 at Darkwood on the mid-north coast.

But so far there have been no reports of any properties being inundated.

Mr Eburn said there has been 18 flood rescues since the wild weather began hitting the state, with people continuing to ignore warnings.

"If people could get the message of not entering flood waters that would really reduce that number," he said.

Calls for assistance, which has now reached more than 2900, are expected to increase as the weather system brings localised downpours of up to 200mm and winds of more than 100km/h to the cities.

There are reports of localised flooding around Camden in Sydney's southwest but that there was no threat for any rivers to flood, he said.

"Don't get out on the road if you don't have to today and stay clear of fallen power lines."