The flood crisis gripping the state has now claimed eight lives after two bodies were found in floodwaters in western Sydney on Tuesday, as the coast is battered by unrelenting torrential rain.
Premier Dominic Perrottet says 40,000 people around NSW are under 59 evacuation orders, including orders in close to 40 suburbs around Greater Sydney.
There are multiple flood warnings for vast swathes of NSW and many roads, bridges and schools are closed.
About 2000 people near Manly Dam will be told to evacuate on Tuesday afternoon.
NSW SES Commissioner Carlene York said the dam is spilling but not at risk of failing.
The order is likely to affect residents along Manly Creek at North Manly and Manly Vale.
Police have called the deaths of two people killed in western Sydney a "tragic reminder" for people to keep off the roads as NSW suffers through dangerous weather.
Detective Superintendent Paul Devaney said the Wentworthville canal where the mother and son were found on Tuesday "rose this morning from ankle deep to above neck height in a matter of minutes".
The torrential rain battering Sydney and subsequent flooding likely played a part in their deaths but an investigation is underway.
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, Supt Devaney said.
Police had earlier issued an appeal to find the pair after their Mazda 3 was found in the stormwater canal on Monday.
The SES received more than 2500 calls for help in 24 hours and undertook about 200 flood rescues, mostly in Sydney.
Sydney has endured 16 consecutive days of rain and no reprieve from the deluge is expected until Wednesday, with no sign of sun until Thursday.
A low pressure system off the mid-north coast has brought torrential rain to the Hunter and Greater Sydney regions with warnings for heavy rainfall, flash flooding and damaging winds for more than half the coast.
The Bureau of Meteorology is warning saturated soil and swollen rivers could lead to landslides and flooding with rainfall of up to 200mm possible in Sydney's west and southwest.
Wind gusts of up to 90 km/h are forecast, stretching south to the Illawarra region into Wednesday.
The bureau's Dean Narramore said it had been a dangerous night in Sydney with widespread heavy rain bringing "devastating flash-flooding to some suburbs".
The Hawkesbury-Nepean is experiencing floods as bad as or worse than those seen in March last year, Mr Narramore said on Tuesday afternoon.
The severity will depend on how much more rain falls in the next 12 hours.
He advised residents stay inside and up to date with flood warnings and evacuation orders.
"But there is some good news, the low pressure system is moving off the coast," Mr Narramore said.
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.
Claims will be processed by urgency, meaning some people with flooded but habitable homes could be waiting weeks to have their claims assessed.
Sydney Trains is expecting significant delays and cancellations on most services due to the severe weather.
Drivers are also being asked to stay off the road unless necessary, with the sustained wet weather opening potholes and causing flash flooding.
The Illawarra Highway was closed on Tuesday between Moss Vale and Robertson in the Southern Highlands after a sinkhole formed under the road at Burrawang, NSW Police said.
NSW LiveTraffic tracker shows close to 400 warnings for flood affected roads.
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