North NSW facing worst floods in 35 years

Thousands of residents in northern NSW are bunkered down in evacuation centres, as the worst floods in over 35 years hit the region.

Across NSW, more than 12,000 people were isolated by rising floodwaters on Thursday as heavy rainfall lashed the state, sparking 13 flood warnings.

Evacuation operations continued in and around Moree late on Thursday afternoon, with about 2300 residents rushing to sandbag their homes and leave before darkness fell.

Helicopters descended on nearby Pallamallawa to airlift some of its 600 flood-stricken residents to safety, with the Gwydir river expected to peak there at 6pm (AEDT).

NSW SES deputy commissioner of operations Steven Pearce said the Pallamallawa operation was challenging.

"But we are very confident we will get everyone to safety," he told AAP.

At Moree, both the Mehi and Gwydir rivers were predicted to peak on Friday morning, matching or exceeding the 10.6m levels reached in the February 1976 flood.

"We're looking at water up to the knees (in some areas)," Moree Mayor Katrina Humphries said in a statement urging affected residents to leave their homes.

About 1600 Moree residents and 80 people from Biniguy were expected to evacuate to south Moree evacuation centres before dark, when the main bridge at Moree would be closed and the town split in two.

"Everyone is really pulling together to help sandbag properties and there's a really strong community spirit," SES spokeswoman on the ground, Heidi Groom, told AAP.

NSW Police and Emergency Services minister Mike Gallacher extended natural disaster declarations on Thursday to the Moree, Narrabri, Gwydir, Tenterfield and Greater Taree local government areas.

"The emergency service personnel deployed to these areas have done an outstanding job helping communities who are in the thick of this weather system," Mr Gallacher said in a statement.

Premier Barry O'Farrell, Deputy Premier Andrew Stoner and Mr Gallacher will tour the flood-affected regions on Friday.

The SES estimates about 12,150 people will be isolated across the state by Thursday night.

These include the 2300 at Moree, 2300 on the mid-north coast, 2500 around Bellingen and 4650 around Harrington at the Manning River, the SES said.

Essential supplies, including blood from Mungindi Hospital just across the border in Queensland, were brought into Moree, while food and supply drops were expected to take place in isolated regions.

Thirteen flood warnings remained in place for river systems across NSW, with rain expected to spread to the Hunter region, the Illawarra and metropolitan Sydney later on Thursday.

"This is a campaign of flood events that are likely to stay around for some time," deputy SES commissioner Pearce said.