Floodwaters in Australia's Queensland state are moving slowly towards New South Wales (NSW) state, with authorities warning small communities south of the border to prepare for the deluge, the Australian Associated Press (AAP) reported on Tuesday.

About 400 people in the towns of Goodooga, Weilmoringle and Angledool in northern NSW are facing the possibility of isolation when flood peaks flow into the state's north later this month.

The State Emergency Service (SES) said that there were no homes under threat but roads would be cut by the floodwater, leaving residents stranded in a handful of communities and surrounding rural properties.

The weather bureau is predicting moderate to major flooding on the Culgoa, Bokhara, Birrie and Narran rivers in northern NSW, swamping the region for four to six weeks.

The Bokhara River at Goodooga is expected to reach 4.2 meters on January 21, while the Culgoa River at Weilmoringle is likely to peak at almost 6 metres on January 23.

"It's weeks away but these are very slow rising floods ... which means roads can be cut a week or more ahead of the flood peak and a week or more after," an SES spokesman told AAP.

"It's not out of the ordinary, these are decent sized floods but they are not exceptionally large like those in Queensland which are floods of record," the spokesman said.

Queensland's flood crisis has inundated or isolated more than 20 towns and cities, with more than 200,000 people affected.