Areas stretching from Newcastle to the South Coast and as far inland as Oberon are at risk, but especially western Sydney's flood-prone Hawkesbury-Nepean area.
NSW Emergency Services Minister Steph Cooke warned affected communities not to take their safety for granted.
"It was a long night last night. It's been a very long day today, and tonight will be another long night for our emergency services workers and our volunteers," she said.
"If you were safe in 2021 do not assume you will be safe tonight. This is a rapidly evolving situation and we could see areas impacted that we haven't seen before."
The NSW SES urged the people to stay safe, and those in flood-prone areas to avoid unnecessary travel as the rain persists.
Assistant Commissioner Dean Storey said 137 flood rescues have taken place and SES members have responded to more than 3000 requests for assistance.
"In many areas it is still an evolving flood situation, and we may see more rain than what was originally forecast," he said.
The wet weather system is expected to move south along the Hunter, Central and Sydney coasts in the next 24 hours.
A coastal trough lingering since Friday deepened while an east coast low-pressure system formed off the Mid North Coast.
The Bureau of Meteorology's Jane Golding said thunderstorms are also expected, potentially delivering more rain and flash floods.
The bureau has issued flood warnings for the Hawkesbury, Nepean and Colo rivers where levels in some parts of these catchments have already exceeded levels recorded earlier this year.
Camden in Sydney's far southwest Sydney will be in for a lashing, Ms Golding said.
"It'll be at its worst tonight."
Many locations have seen up to 200 millimetres of rain and some close to 300mm over the last 24 hours.
The bureau warned river levels could reach those of March this year when swathes of Sydney's southwest and outer west were inundated.
Warragamba Dam in Sydney's west began overflowing at 2am, well ahead of predictions.
The dam was spilling at a rate of 500 gigalitres (a day), Ms Cooke tweeted on Sunday evening, which is higher than during the floods this year which inundated large swathes of the state.
"To give some context, this rate is greater than the March 2021 flooding period, March and April 2022," she told reporters on Sunday evening.
Meanwhile, a man has died while kayaking in Sydney Harbour near Canada Bay on Sunday, but police are still investigating and have not linked the incident to the weather.
Evacuation centres are open in western Sydney at Canley Vale's Cabra-Vale Diggers Club, the Narellan community centre, Gymea Tradies Club, Richmond Club, North Richmond community centre and Castle Hill RSL.
The SES was joined by 100 Australian Defence Force members on Sunday, assisting with sandbagging and doorknocking communities to warn of flood threats focusing on the Hawkesbury-Nepean area.
EVACUATION ORDERS IN PLACE IN PARTS OF:
* Agnes Banks, Angus, Gronos Point, Plough and Harrow Dam near Abbotsbury and Cecil Hills, Pleasure Point, Bents Basin, Wallacia, Camden, Woronora, Chipping Norton, Georges Hall, Lansvale, Moorebank, Warwick Farm, Leets Vale, Ebenezer, Pitt Town, Pitt Town Bottoms, North Richmond, Sackville, Cattai, Cornwallis, Richmond Lowlands, South Windsor, Bligh Park, Freemans Reach
EVACUATION WARNINGS IN PLACE IN PARTS OF:
* East Hills, Picnic Point, Penrith, Emu Plains, Mulgoa, Jamisontown, Cranebrook, Yarramundi, South Maroota
MODERATE TO MAJOR FLOODING POSSIBLE ALONG:
* Hawkesbury and Nepean Rivers, Colo River, Georges River and Shoalhaven River.
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