3500 told to evacuate in flood-threatened NSW

About 2000 people in Sydney's flood-threatened north-western fringes have been told to evacuate as emergency services battle what they say is a record event.

The evacuation order, issued on Saturday morning, takes the total number of people told to leave their homes across NSW to 3500.

NSW State Emergency Services commissioner Murray Kear told reporters the Hawkesbury-Nepean River was currently at seven metres and rising as water spilled from the overflowing Warragamba Dam.

The river is expected to peak at up to 11 metres over the next 48 hours.

"We would have to go back 20 or 30 years to see some similar peaks," Mr Kear said at the SES south-western headquarters in Sydney on Saturday.

"There has been an unprecedented amount of rain over a long duration.

"This rain event covers 75 per cent of the state."

The Transport Management Centre said the North Richmond, Windsor, Yarramundi and Cattai Bridges were all now closed.

"Motorists are no longer able to drive to Richmond along Bells Line of Road and Kurrajong Road, or to Windsor via Putty Road/Singleton Road and Wilberforce Road," the centre said in a statement.

"Motorists on the western side of the Hawkesbury River can return to Sydney using the Bells Line of Road (westbound), Darling Causeway (from Bell to Mount Victoria) and the Great Western Highway (eastbound) back to Penrith."

Mr Kear said people needed to be prepared for flash flooding, with another 200 millimetres of rain set to fall across the state over the weekend.

"This could lead to homes being isolated or inundated by water," he said.

About 500 SES volunteers are working throughout the state, with more than 43 flood rescues carried out since Monday, including two people rescued overnight after they were trapped in their car at a river crossing at Blayney, near Bathurst in the NSW central west.

Mr Kear urged people to stay clear of flood waters and heed emergency warnings.

"If you do go out, that means that the lives and the time of emergency services people will be at risk," he said.

A torrent of water was released from Warragamba Dam on Friday night after it reached full capacity for the first time in 14 years.

The SES said the bridges at North Richmond and Windsor were expected to remain closed for the next 48 hours.

Flood evacuation orders have been issued for areas around Pitt Town Bottoms, Richmond Lowlands and Grono Point as well as caravan parks in low-lying areas along the Hawkesbury River from Windsor to Wisemans Ferry.

-AAP