Heavy rain has caused flooding on the NSW north coast with some people being rescued from floodwaters as rivers swell after thunderstorms overnight.

The Bureau of Meteorology said moderate to heavy rainfall associated with thunderstorms has been recorded across the Orara River catchment in the Coffs Harbour region, affecting the towns of Glenreagh and Coutts Crossing.

Coffs Harbour State Emergency Service deputy unit commander Martin Wells said several people had to be rescued from flooded roads overnight.

"It's really disappointing that drivers continue to head through these flood waters when it's clearly impassable," he told radio 2GB on Thursday.

"It's putting theirs and their family's lives at risk and of course our teams that are heading out to rescue them from those situations," he said.

The SES is warning Glenreagh Village may be isolated on Thursday if river levels continue to rise and residents should prepare for possible isolation.

It's also likely that all road and bridges to and from Glenreagh will be impassable once the river exceeds 7.4 metres at the Glenreagh gauge.

The Kremnos and Kungala area may be isolated and residents should prepare as it's likely that all evacuation routes from those towns will be impassable once the river exceeds 5.8 metres at the Glenreagh gauge.

The SES said people should refrain from driving or walking through floodwater, stay out of rising water and seek refuge in the highest available place.

© AAP 2021