Flood warnings remain in place for some 14 rivers in northern NSW.

The State Emergency Service (SES) says that the number of people cut off is expected to increase, as more rain falls on Tuesday.

Areas around Grafton in the Clarence River Valley in northern NSW, 630 kilometers north of Sydney, are of most concern to the SES on Tuesday, SES spokesman Phil Campbell said.

The Clarence River has swollen to 4.2 meters at Grafton on Tuesday morning and is expected to peak at 6.5 meters early on Wednesday, potentially cutting the Pacific Highway and requiring the evacuation of caravan parks.

Further north in the Clarence River valley, 500 people are trapped at Bonalbo, about 200 at Urbenville and another 300 at Tabulam.

To the south, at Darkwood on the Upper Bellinger River more than 500 people are cut off by floodwaters.

The town of Bellingen is cut in two and may be isolated later on Tuesday.