Thousands evacuate as Sydney rivers rise

Scores of evacuation orders and warnings have been declared, mostly northwest of Sydney, where major flooding is occurring along the Hawkesbury and Nepean rivers at Menangle, North Richmond, Lower Portland and Windsor.

Residents in parts of Chipping Norton in Sydney's southwest have been directed to evacuate by 1145 pm, as floodwaters rise at Milperra.

Flood warnings are in place for the Georges and Woronora rivers.

The SES is warning that people who don't leave when directed may become trapped without power, water and other essential services and it may be too dangerous to rescue them..

Evacuation centres have been set up across western Sydney at Canley Vale, Narellan, Gymea, Richmond, North Richmond and Castle Hill, and at Gosford on the Central Coast.

Premier Dominic Perrottet says the latest inundation could threaten more people and properties than previous floods.

"Experiences in the past do not necessarily mean that the flood event will be the same ... there could be a worse situation," Mr Perrottet said.

He urged people to follow SES orders.

The State Emergency Services had received more than 3900 requests for help and conducted 85 rescues to 3pm on Monday.

"We anticipate another busy night," SES Commissioner Carlene York told the ABC.

At Revesby Heights a Fire and Rescue NSW crew saved a family of five, including a pregnant woman, trapped in floodwaters on Monday night.

Firefighters came to the aid of the family whose car had become stranded near the corner The River Road and Prince Street. No one was hurt in the incident.

Rain is forecast for the rest of the week and the flood risk will remain even after the wet weather stops, with saturated catchments likely to react quickly to any falls in the coming weeks.

There are severe weather warnings of damaging winds, hazardous surf and heavy rain that may cause flash flooding in the Illawarra, Blue Mountains, Sydney Metropolitan and parts of Hunter and Central Coast districts.

NSW has requested another 100 Australian Defence Force members, on top of 100 who joined on Sunday, helping with sandbagging and doorknocking.

Federal Emergency Management Minister Murray Watt said he was on standby to help NSW, with Prime Minister Anthony Albanese and Mr Perrottet discussing the situation overnight.

"We want to be very quick and thorough when it comes to providing recovery support to people as well," Mr Watt told the ABC.

Flood Recovery Minister Steph Cooke said communities will get help to recover from yet another flood when disaster declarations are made.

"For many communities this is the fourth flood that they have seen in less than 18 months," Ms Cooke said.

Camden Council deputy mayor Paul Farrow said the southwest Sydney community was heartbroken and fatigued by repeated flooding over the past 18 months.

Risk Frontiers resilience general manager Andrew Gissing said community flood recovery can be hindered by repeat events.

"They are forced to continually pick up the pieces," he said.

Mitigation strategies and encouraging development in lower risk areas could reduce flooding in the future, he said.

Warragamba Dam began spilling on Sunday morning, and by evening water was gushing over the wall at a rate of 515 gigalitres per day.

WaterNSW said the flow slowed overnight, down to 380GL/day, but the spill will depend on rain in the catchment on Monday.

The dam flows into the swollen Hawkesbury and Nepean rivers, which are above major flood levels in multiple areas.

Farmers have called for the dam wall to be raised to prevent floods damaging agricultural land.

NSW Farmers president James Jackson said the Sydney basin grows up to $1 billion of produce annually.

"How many floods do we have to have before we realise dams won't stop a flood but they can change the shape of it and reduce the damage significantly?" Mr Jackson said.

"It's very fast moving water, it rips the ground away and destroys their crops."