A severe weather system heading south along the NSW coast is only going to get more intense, authorities warn.
King tides, winds up to 125km/h and up to 200mm of rain are expected to hit Newcastle and Sydney as a wild east coast system makes its way south overnight.
The NSW State Emergency Service has already issued a flood evacuation warning for residents at Fingal Head, south of Tweed Heads.
An evacuation centre has been opened as a precaution because of fears the Tweed River could breach its banks.
Flood watches are also in place for other rivers on the east coast, including warnings for the Hawkesbury and Nepean Rivers in low-lying areas of western Sydney, including around Richmond and Windsor.
"Once this weather system hits Sydney, a main focus now is particularly western Sydney," SES spokeswoman Stephanie Sullivan told AAP.
"We're expecting 150-200mm around Sydney overnight, really high figures considering we've got high tides."
Storm damage in Sydney. Source: Outerinnerwest.com
The worst of the weather is expected to hit Newcastle and Sydney on Saturday night and into Sunday and could cause flash floodings, prompting warnings for people to take extra precautions to stay safe.
Five people have already been rescued after their car got caught in flash floodwaters at Murwillumbah in the state's north.
"Our major concern right now is making sure everyone realises this is going to be more intense - it's intensifying from what we're seeing now," Ms Sullivan said late on Saturday afternoon.
Bureau of Meteorology senior meteorologist Adam Morgan said it's unusual to see an east coast low tracking southward, and that it had brought about an unusual warning situation.
"We've got forecasters in NSW that can't remember in 30 years having a flood watch out for the entire NSW coast," he said.
As well as heavy rainfall up to 200mm, localised rainfall could reach 300mm in some areas.
The weather has already knocked out power to parts of Sydney and the Central Coast.
Ausgrid said emergency crews were working on Saturday afternoon to restore power to about 800 customers across multiple locations in those regions.
Sydneysiders planning on visiting the Vivid light festival have been advised to postpone their trips, with the majority of lighting installations around Circular Quay and Martin Place cancelled because of public safety fears.
Abnormally high tides are also predicted by the weather service, causing sea water to flood low-lying areas - and coastal wave heights could reach as high as 15 metres in some areas.
"Everywhere is a bit of a risk, but probably the far north and the far south coasts tomorrow in terms of rain are the bigger risks for those higher-end falls," Mr Morgan said.
The SES received almost 300 calls for emergency assistance in the 24 hours to 8am Saturday, mostly from residents in the state's north but also Port Macquarie, Coffs Harbour, Maitland and Gosford.
Flood watch warnings have been issued for river valleys in large parts of eastern NSW, with flash flooding likely.
