Thousands still isolated by NSW floods

More than 20,000 people remain isolated or homeless due to northern NSW floodwaters, with authorities saying some may be cut off for three weeks.

The State Emergency Service (SES) and the NSW Police are also warning of fraudsters looking to use the flood devastation to scam money from generous citizens.

River levels on the far north and mid-north coasts continued to fall below moderate and minor flood levels on Tuesday.

The total damage bill is not expected to be known until the water completely recedes, but the Insurance Council of Australia estimates it will top $39 million, including the affected areas in southeast Queensland.

However, moderate to minor flood warnings from the Bureau of Meteorology (BoM) remain for the Richmond and Wilsons rivers in the Lismore area, the Clarence River in the Grafton and Maclean areas, the Bellinger river at Bellingen and the Macleay River in the Kempsey area.

The SES has received more than 3,000 calls for assistance since storms wreaked havoc on the Tweed/Lismore area early last week and then moved south.

More than 20,000 residents in the affected areas remain isolated within their properties or cannot return home after evacuating flood-prone districts.

A SES spokeswoman said access would be restored to many areas within the next two days but pockets of land will remain cut off for much longer.

"While access is expected to be restored over the next couple of days, some of these may be isolated between one to three weeks," she told AAP.

The SES and NSW Police say no official flood appeals have been set up to assist flood victims.

It comes after a woman reported she gave her credit card details to a man who phoned pretending to be collecting for flood victims on Monday, police say.

The SES said in a second incident, two men in an unmarked vehicle have been claiming to be doorknocking on behalf of SES volunteers.

"SES volunteers do not collect money during operational periods," a statement from the organisation said.

The Lismore, Byron, Ballina, Kyogle, Richmond Valley and Tweed local councils have established a joint disaster recovery committee to coordinate the clean-up efforts.

Lismore Mayor Jenny Dowell heads the committee and says total damage has been estimated in the tens of millions of dollars.

"Council crews and others are looking around all the local government areas and counting the number of road washouts, land slips, bridges that have gone and other damage to infrastructure," Ms Dowell told AAP.

"And we are slowly getting a picture of what each council is facing."

"The coastal councils of course have major concerns about beach erosion and we're facing the possibility of significant impacts to our primary industries."