Thousands of residents have been evacuated from their northern NSW homes as emergency crews brace for further flooding when rivers hit their peak later today.

The State Emergency Service (SES) has ordered residents to evacuate north and south Lismore, and has also told business owners and residents to leave the CBD.

About 3000 people have so far been moved to higher ground in the town - down on an earlier estimate of 6000 - SES spokesman Steve Delaney said.

The Wilson River, which runs through the heart of Lismore, is expected to continue to swell with a peak expected later today.

Mr Delaney said Lismore residents were "very flood aware" and many were already prepared for the evacuation.

"They have been preparing all through the night basically raising all their goods above water level and moving up into the higher areas of their homes," he said.

"So we just want to make sure that those who need to move out actually move out and do it safely.

"The weather up there is still very, very bad and the rain is coming in sideways and there is a fair wind blowing."

Police have warned people to stay off the roads and not to attempt crossing flooded causeways.

The Pacific Highway was blocked 10km north of Woolgoolga and north of Ballina, and major arterial roads in the Tweed-Byron area were also affected by flooding, police said.

"Police strongly urge people to remain at home and undertake only essential journeys," a police spokesman said.

Earlier today, more than 75 elderly residents at a nursing home in Benora, on the Queensland border, were asked to evacuate and about 50 people in a Lismore caravan park were being moved to higher ground.

Several people have been rescued, including a 50-year-old man plucked from a creek near Lismore by emergency volunteers using a front-end loader.

The storm system has so far dumped more than 100mm in 24 hours on parts of NSW's north, including Coffs Harbour, Grafton and Lismore.

In Queensland, most Gold Coast suburbs are flooded and emergency services are searching for a couple feared swept from a flooded causeway there.

Downstream of Lismore, moderate to major flooding has been forecast for later today around the Richmond River at Coraki, Bungawalbyn and Woodburn.

Minor flooding has also been forecast for the Brunswick River at Mullumbimby and Bellinger River through Bellingen.

Last night, residents in Yamba and Byron Bay were sandbagging their homes as flash flooding hit the towns.

The Bureau of Meteorology has forecast more rain for NSW and Queensland today.

Flash flood warnings are current for the Northern Rivers, mid-north coast and the Northern Tablelands.

A Sydney Catchment Authority spokeswoman said a slight increase in dam levels was expected when the figures were released at 3pm (AEST).