The state has been hit by repeated flooding in recent months, with the Northern Rivers area devastated by two deluges within weeks and Sydney drenched in its wettest March on record.
A severe weather warning is in place on Thursday for southern and central NSW, metropolitan Sydney, the Illawarra, the South Coast, the Central and Southern Tablelands, and parts of the Hunter.
Severe thunderstorms are also predicted inland for the Central West Slopes and Plains near Parkes and the Upper West near Cobar.
Heavy rain fell in the Illawarra, South Coast and Southern Tablelands overnight and the wet weather will extend across Sydney, the Central Tablelands and Hunter region on Thursday.
Six-hourly totals between 60mm and 100mm are forecast, with up to 140mm predicted on the coast.
Cronulla recorded 107mm in three hours overnight.
Numerous areas around Sydney and the Illawarra have had more than 100mm since 9am on Wednesday.
Significant falls were recorded around Sydney at Rose Bay (140mm), Little Bay (140mm), Belrose (117mm) and Marrickville (110mm), and further south in the Illawarra and South Coast at Darkes Forest (193mm), Macquarie Pass (184mm), Bodalla (127mm) and Nowra (125mm).
The Bureau of Meteorology warns there is an increased risk of landslides.
A flood watch has been issued for central NSW, with minor to moderate flooding forecast for the southern coastal rivers including the Hawkesbury-Nepean, the Macquarie and the Queanbeyan on Thursday and Friday.
On Thursday morning, the SES warned there could be major flooding in Liverpool and Milperra in Sydney's west along the Georges River, with the water expected to rise above four metres in the late-afternoon high tide.
"Residents of areas expecting to be flooded should make plans to leave when advised to do so. Ensure you take pets and valuables with you," the SES said.
Minor flood warnings were issued for the Hawkesbury River at Windsor and North Richmond, the Cooks River at Tempe Bridge and the Woronora River at Woronora Bridge.
Liverpool Mayor Ned Mannoun told Sydney radio 2GB on Thursday there was a sense of deja vu in the area following flooding in early March.
Residents are being invited to pick up sandbags.
"We know what's going to happen, we just hope it won't get worse than last time," Mr Mannoun said.
Moderate flooding could occur on the Colo River at Putty Road.
SES Assistant Commissioner Dean Storey said the SES had conducted 11 rescues and responded to almost 600 requests for help in the past 24 hours.
"We're expecting those numbers to increase unfortunately today as that rain continues," he told Nine's Today program on Thursday.
"It's a very dynamic and volatile situation."
Catchments were already soaked after months of heavy rain, the BOM said.
The bad weather is being driven by a strong upper trough over the centre of NSW, working to deepen another trough sitting off the coast.
The systems are expected to weaken on Friday morning.
"Heavy and persistent showers over the coming days will increase the chance of flash flooding and landslips over already saturated catchments," bureau meteorologist Sarah Scully said.
Severe thunderstorms also pose a threat, including in northeastern NSW.
"They may produce localised heavy falls (but) it is not expected to produce that riverine flooding," Ms Scully said.
"Instead, it will be more localised flash flooding."
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