The city of Queanbeyan, south-east of Canberra, has been declared a natural disaster zone after experiencing its worst flooding in decades.

The Queanbeyan River peaked at 8.4 metres leaving the southern New South Wales city's centre underwater.

The flood waters rose fast and furiously with the river breaking its banks in several places.

State Emergency Service spokesman Tony Casey says floodwaters are receding.

"From the noon peak it's gone down 0.7 of a metre in three hours which is really encouraging and hopefully that drop will continue throughout the rest of the afternoon and into the evening," he said.

Mr Casey says it is too early to say when evacuated residents will be able to return to their homes.

"I wouldn't like to predict a time. The Bureau [of Meteorology] said it will be a long slow decline from the peak," he said.

"There's lots of debris, logs and stuff around, so we've got to make sure conditions are safe for people to be able to use all of the roads that are closed currently and the debris is removed."

Emergency Services Minister Steve Whan, who lives in Queanbeyan, says the city and neighbouring Palerang Shire have been declared natural disaster areas.

"That brings to 30 the number of shires to be declared in this event around NSW," he said.

"That's a fairly unprecedented number to have right up and down the state and it means we do have a big clean-up ahead of us."

About 100 homes and businesses have been evacuated in Trinculo Place, Thorpe Avenue and Antill, Collett, Monaro, Wanniassa, Atkinson, Crawford and Carinya streets.

Many businesses have suffered significant water damage. Some witnesses have told the ABC the waters rose so quickly it was difficult to salvage their belongings.

Queanbeyan Mayor Tim Overall says it is too early to say what the clean-up bill will be.

Australian Defence Force personel have been called in to help SES crews and Rural Fire Service volunteers with the clean-up.

The SES has rescued 10 people, ferrying them to higher ground by boat.

A crane had to be brought in to rescue caravans from the Riverside Caravan Park, while a helicopter was called to help a pregnant woman who was stranded.

Many riverside residents have heeded the warnings and moved to evacuation centres at the Queanbeyan TAFE and the Queanbeyan High School hall.

But some people are simply enjoying the spectacle.

Queanbeyan had been in drought for many years and the overnight deluge and subsequent damage have stunned some locals who have not seen such flooding since the mid 1970s.

Julie Brennan from the NSW Education Department says all schools in Queanbeyan are being closed.

"We're asking parents, when it's safe to do so, to collect their children from all Queanbeyan schools," she said.

More rain is forecast to hit the capital region this evening.

Captains Flat

The Palerang Shire Council says it is still working to open the flood gates of the Captains Flat dam south of Canberra.

Shire chief executive Peter Bascomb says metal walkways on the dam have been damaged and they are having difficulty opening the gates.

Residents below the dam and along the Molonglo River are being told to remain ready for evacuation at short notice.

"People still need to be mindful and listen out for any advice that we give," Mr Casey said.

"But the Molonglo River is dropping fairly quickly, so the situation is a lot better than it was this morning."

Shelter is being provided at Captains Flat Primary School.

Canberra flooding

The ACT SES has responded to more than 170 calls for help across Canberra since midnight, with the Tuggeranong Valley the worst affected by the wild weather.

More than 90 millimetres fell in Tuggeranong overnight, three times the amount that fell at the airport.

Most calls for help have been for flooding in and around homes and businesses.

Major roads around Canberra are closed or partly blocked, including parts of Lady Denman Drive, Majura Road and Flemington Road.