Record floods stranded more than 50,000 people in eastern Australia on Thursday, killing three as a muddy tide swept through towns and swollen rivers cut off roads.

Police have pulled three bodies from rising floodwaters on the Mid North Coast, a river-braided region of rugged hills and fertile valleys about 400 kilometres north of Sydney.

Authorities launched a major search-and-rescue mission as people clambered atop cars, houses and highway bridges to escape fast-moving flash floods.

The storms have dumped more than half a year's worth of rain over just three days, the government weather bureau said, smashing flood-height records in some areas.

"We are seeing levels in local tributaries, creeks and rivers that we haven't seen since 1920," New South Wales (NSW) state premier Chris Minns told reporters.

"Many people will have never seen this level of inundation or flooding in their communities."

The town of Kempsey, a farming hub on the banks of the Macleay River, had been cut off with little warning, said Mayor Kinne Ring.

"You often think of rain on a tin roof as relaxing, but at the moment it is deafening and horrible," Ring said.

"The downpours are torrential and every time it rains, you wonder what is going to happen next."

Ring said more than 20,000 people were isolated in her local government area alone.

About 140 kilometres south in the town of Taree, business owner Jeremy Thornton said the "gut-wrenching" flood was among the worst he had seen.

"It is pretty tough, we've had a few moments but you have to suck it up and push on," he said.

"We are reliving it every second – hearing the rain, hearing the helicopters, hearing the siren."

Authorities said more than 50,000 people were cut off, with some rivers still to reach peak levels late on Thursday.

An elderly couple climbed on to the roof of their car to escape a fast-rising flash flood before a rescue helicopter winched them to safety, NSW police in Taree said in a statement.

Others sought sanctuary on a raised highway bridge before they were spotted and rescued by a navy Seahawk chopper.

Locals spotted dead cows washing up on beaches after swollen rivers swept them from their pastures inland.

The government has declared the emergency a natural disaster, unlocking greater resources for affected areas.

Police said they were still searching for one person reported missing. (AFP)