Residents of multiple areas that experienced flooding 12 months ago have been warned rivers could rise overnight and deliver worse conditions this time around.
Premier Dominic Perrottet says 40,000 people around NSW are under evacuation orders, including in close to 40 suburbs around Greater Sydney.
There are multiple flood warnings for vast swathes of the state, with many roads, bridges and schools closed.
About 2000 people in low-lying areas near Manly Dam in Sydney's north were warned to prepare to evacuate as the dam began spilling on Tuesday.
Northern Beaches Council Mayor Michael Regan told ABC News relocation centres had been set up in anticipation at the former Seaforth TAFE, and Freshwater surf club.
The SES received more than 2500 calls for help in 24 hours and undertook about 200 flood rescues, mostly in Sydney.
The city has endured 16 consecutive days of rain and no reprieve from the deluge is expected until Wednesday, with no sign of sun until Thursday.
Wind gusts of up to 90 km/h are forecast, stretching south to the Illawarra region into Wednesday, and there is a risk the winds could topple trees and down powerlines.
The Hawkesbury-Nepean catchment in Sydney's west is experiencing floods as bad as or worse than those seen in March last year, the Bureau of Meteorology's Dean Narramore said on Tuesday afternoon.
The severity will depend on how much more rain falls in the next 12 hours.
Flooding worse than that of last year was already occurring on Tuesday afternoon in western Sydney on the Upper Nepean River at Menangle and the Hawkesbury River at Lower Portland and Wisemans Ferry.
Worse than 2021 levels are also possible at North Richmond, Windsor and Sackville, the BOM warned.
The Hawkesbury could reach as high as 14.2 metres at North Richmond by Wednesday morning, with further rises possible along the river.
In the Hunter region, Bulga southwest of Singleton could see worse floods than last year on Wednesday morning, with major flooding likely to hit Singleton as well.
Several areas around the town were warned on Tuesday they needed to prepare to evacuate.
The deaths of two people killed in western Sydney have been called a "tragic reminder" for people to keep off the roads.
NSW Police Detective Superintendent Paul Devaney said the Wentworthville canal, where the bodies of a mother and son were found on Tuesday, "rose this morning from ankle deep to above neck height in a matter of minutes".
The victims are believed to be 67-year-old Hemalathasolhyr Satchithananthamand and her 34-year-old son Bramooth who had been missing in the area after their car was found in the stormwater canal on Monday.
Efforts to recover the bodies were hampered by the weather in an operation that involved police divers, the rescue squad, assistance from the State Emergency Service and PolAir.
The first body was found about 1.4 kilometres from the car, and the second about 900 metres further away.
Footage shared to social media as rain lashed Sydney has shown water pooling in the Harbour Tunnel, parts of the shopping centre roof collapsing at Bondi Westfield, and cars trapped as heavy rain flooded parts of the Roseville Bridge.
More than 30,000 insurance claims have been filed around NSW, the Insurance Council of Australia said, with the expected cost of the floods in Queensland and NSW now approaching $1.5 billion.
Some could be waiting weeks to have their claims assessed.
