Moorebank, Chipping Norton, Holsworthy, Picnic Point, Pleasure Point and Sandy Point residents were ordered to evacuate on Sunday night after more than 300 millimetres of rain lashed the region over the weekend.
The NSW State Emergency Service said people risked being trapped without power and water if they didn't leave the low lying areas along the Georges River.
The affected area is for all properties on Arthur Street (off Rickard Road), Rickard Road, Davy Robinson Drive and Newbridge Road between Milperra Bridge and Riverside Road.
In Holsworthy, residents on Heathcote Road, between Harris Creek and Deadmans Creek, are also being evacuated.
The order is also in place for properties on Carinya Road and Amberdale Road (Picnic Point), the Northern side of Riverview Road (Pleasure Point) and St George Crescent and Gambier Street (Sandy Point).
An evacuation centre has been established at Dunc Gray Velodrone.
The Georges River at Liverpool Bridge is estimated to have peaked near 5.40 metres with major flooding. Several roads across the area have been closed.
Emergency services have been swamped with calls since the deluge set in on Friday, while the extreme weather has caused transport chaos across Sydney. The Badgerys Creek weather observation station recorded 328 millimetres of rain between 7am Friday and 10.30pm on Sunday, causing sporting fields and roads to be flooded.
The Liverpool SES received more than 115 requests for assistance mainly related to downed trees and roof damage between Friday and Sunday morning.
"We have been absolutely hammered by this crazy weather today resulting in trees giving up and taking a dive, many taking out powerlines on their way down," they wrote on their Facebook page.
"If you see fallen powerlines always assume they are live and stay at least 8-metres away.
"Some roads have also been blocked by fallen trees, other roads have been affected by flooding. Stay safe Liverpool - never drive, ride, walk or play in floodwater!"
A severe weather warning was issued on Sunday for the entire coast of NSW, and will remain in place on Monday.
The Bureau of Meteorology says the deluge could cause "life-threatening flash flooding" in the Hunter region, the Central Coast, greater Sydney, Illawarra and the Blue Mountains.
The last time Sydney and the state's coast had as much rainfall was mid-2016.
"But we've surpassed those figures and you have to go back as far back as 1998 to see totals like we'll get," a weather bureau spokeswoman said.
NSW Police Assistant Commissioner Karen Webb urged drivers to take caution and avoid floodwaters.
"There are currently hundreds of calls for assistance, including trees, boulders or power poles down onto cars and homes, and across roads, as well as power outages and localised flooding impacting various roads and traffic lights," she said.
"I'm disappointed that I need to remind people to act responsibly and not to take risks in these types of conditions, especially when around floodwaters."
Dams around Sydney, including the major Warragamba Dam, are swelling to their highest levels in years.