Tens of thousands of residents across a swathe of south-western Sydney suburbs have been told to evacuate immediately due to flooding concerns.

The State Emergency Service has issued evacuation orders for the following locations: Chipping Norton, Georges Hall, Picnic Point, Pleasure Point, Sandy Point and parts of Lansvale, Holsworthy, Milperra, Moorebank and Warwick Farm.

SES spokesperson Sandie Smythe said the situation has developed rapidly, with authorities given no time to issue warnings and moving straight to evacuation orders.

"We've had a mass amount of rain that's fallen in an already saturated and previously flooded catchment so it's been a quick and emerging situation and we have progressed to evacuation orders very quickly," she told the ABC.

"It is a very extensive area all affected by this Georges River system."

The SES has sent people text messages alerting them to the evacuation orders and is also ringing people's landlines, and emergency service crews have also been door-knocking homes in some areas, telling people to leave immediately.

Authorities are urging residents to stay with friends or family or find alternative accommodation outside the flood areas.

Evacuation centres have also been set up at Cabra-Vale Diggers Club in Canley Vale and Club Menai in Menai for those who need it.

It comes after evacuation orders were also issued for parts of Camden, also in Sydney's south-west, with residents in some areas told to leave by 9.00pm on Monday night and others advised to be out by midnight at the latest.

The SES says if people don't heed the evacuation orders they risk being cut off by rising floodwaters.

The latest evacuation orders come as a new east coast low bears down on the state and the weather bureau warned of more heavy rain, flooding and possible landslides for large parts of coastal New South Wales and inland as far as the Blue Mountains.

NSW Emergency Service Commissioner Carlene York said the state was facing a "very serious" situation on Monday night and into Tuesday, and urged people to prepare and follow the advice of the emergency services.

"It's important if we give you an evacuation warning to ensure that you have all your essentials and important documentation and are able to leave your premises in a safe way," she said.

It's also important if we put out evacuation orders that you actually leave your premises," she said.