The heavy rain covering eastern parts of Sydney is expected to intensify and move across to other areas over the next 24 to 48 hours in a deterioration of conditions.
Potential flash flooding and overflowing of catchments from Newcastle to Shoalhaven on the state's south has also been predicted.
Of major concern is the Hawkesbury-Nepean river system, which could peak overnight
The State Emergency Service sent text messages to residents of Menangle, Liverpool, Milperra, Camden, North Richmond, Wallacia, Penrith, Sackville, Upper Colo and Windsor warning them to be prepared in case an evacuation order was issued.
Doorknocking took place late in the afternoon in the Picton area. Warragamba Dam was also at risk of spilling.
In the past 24 hours, the Illawarra was pummelled with some areas receiving 200mm of rain in one day — a stark contrast to the July average of about 50mm.
Minister for Emergency Services Steph Cooke said the developing east coast low was making the coastline and cities dangerous places, and "we are all nervously waiting to see what eventuates".
Ms Cooke said she was confident all emergency services were in place to get the community through "in one piece".
"We have helicopters, Unimogs [rough-terrain trucks] and other assets strategically placed to ensure they can respond quickly in the event of an emergency," she said.
The Bureau of Meteorology (BOM) said the trough was just starting to deepen driven by the amount of moisture in the atmosphere.
"There'll be an escalation of the weather, a deterioration of the weather," meteorologist Jane Golding said.
"So the rainfall rates will increase. We'll start seeing the wind increase as well. We’ll start to see the seas whipped up and we'll see rivers respond to the rain that's falling.
"It's a dangerous weather system."
The State Emergency Service (SES) has 147 personnel on duty 24/7.
In the past 24 hours up to 7pm on Saturday, they had responded to 739 requests for help and carried out seven rescues.
Five were in the Illawarra region and two in Greater Sydney.
SES superintendent Tom Jory said the drivers were caught by flash flooding.
"We've seen one where a person has attempted to cross a flooded road," Mr Jory said.
"And we have seen another one where a couple of people have identified that they have become isolated as a low-level crossing has seen a rise of water over the top of it.
"They made the right call, the safe call, and they have called for help before they tried to make their own way across."
Marine Rescue Services, Fire and Rescue NSW and Surf Lifesaving are among the organisations on stand-by.
Some of the areas under flood watch include the Wollombi Brook in the Hunter district, the Hawkesbury-Nepean as well as the Georges, Woronora and Cooks rivers, and the Shoalhaven and Saint Georges Basin in the Illawarra.
NSW SES Commissioner Carlene York said that while units have been pre-deployed to these areas most at risk, the community had a role in keeping themselves safe.
People out camping or away from home for the school holidays needed to take extra care.
"Some people may be away from their home, so remind you to try and get back to family and friends," Ms York said.
"A lot of these areas are low-lying so if you're looking at school holidays, camping sites, caravan parks — anywhere near the rivers are at particularly high risk."
Feds step up
Earlier on Saturday, Federal Emergency Management Minister Senator Murray Watt said the Albanese government was prepared, putting the Australian Defence Force (ADF) on stand-by.
ADF personnel will be deployed from Holsworthy.
"The New South Wales government have requested that we provide two night-time Defence Force helicopters with special equipment particularly for potential rescues and they have also requested that we provide 100 troops," he said.
"The helicopters will be ready from [Saturday] evening, the troops will be ready from [Sunday] morning.
"We all hope these resources won't be needed."
Ms Cooke said her discussions with the minister were ongoing but welcomed the decision.
"Some of the things that those Defence personnel could do would be to assist with door knocking or sandbagging," she asid.
"Or other of those preparation activities that we just may need some additional boots on the ground support.
"So very, very grateful to the Feds for helping us with that, for being ready … look forward to continuing that relationship into the future with the common goal of ensuring that we keep our community safe."
David Williamson, his partner Michelle and their two sons were out in the rain at Sydney Harbour this morning.
"We're from Melbourne on a holiday. I think Sydney rains more than Melbourne these days," Mr Williamson said.
"I think there's 100 per cent [chance of] rain for the five days we're here," Ms Williamson added.