Firefighters were responding to an alarm at the Moree hospital just before midnight on Wednesday when they heard screams nearby, and rushed to help.

The Narrabri Fire and Rescue NSW crew - who have been deployed to the town - found three people trapped inside a sinkhole near the swollen Mehi River.

Zone commander Superintendent Tom Cooper said the trio could have died if the water had come any closer or if emergency services hadn't found them.

"Crews investigated and found three people had come down to the river to have a look at the height of the floodwater when they had been swallowed up by a sinkhole," he told the Leader.

"Fire and Rescue NSW crews got to work with rescue gear ... one man had managed to clamber out of the sinkhole on his own and fire crews rescued two people, a man and a woman believed to be aged in their 40s, from that sinkhole."

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Superintendent Cooper said the soaking wet ground appeared to have "collapsed" under their feet.

He said one of the firefighters described the people as lucky to be alive, let alone escape without any serious injuries.

"It further highlights the dangers of an emergency like this," Superintendent Cooper said.

Moree has been cut in half by a major flood in the Mehi River, which peaked at close to 10.43m on Thursday morning.

The State Emergency Service (SES) said the mammoth clean-up effort can only begin once the water recedes.

Rural Fire Service crews and Fire and Rescue NSW teams are supporting the SES in Moree as the crisis continues.

"We need to assess the damage and get into a recovery operation which will involve making sure people's homes are safe to go back into," Superintendent Cooper said.

"We are waiting for water to recede so we don't do damage by driving trucks through water and creating waves near homes.

"It's going to be a big operation and we have to pay particular attention to anything that may be in the floodwater like hazardous materials in the form of raw sewerage, dead animals and with the mouse plague we are noting that there could be rodent pesticides in the water so we are formulating a plan now."

Hundreds of people were ordered to evacuate their homes earlier this week as the major flood threatened and an evacuation centre is still active at the Moree PCYC.

The latest update from the SES indicates the Mehi River level is now falling at Moree, but it could take some time before evacuated locals can get to their affected homes, and roads reopen.

Roads in and out of Moree are still underwater and closed to traffic, and the bridge joining north and south Moree is also closed.

Emergency services are stationed on both sides of the river to give full coverage to the affected areas.

SES spokesman David Rankine said helicopters and flood rescue boats remain stationed in Moree to help community members flee floodwaters if needed and to deliver essential supplies to the island city.