The Bureau of Meteorology says the heavy rain may lead to flash flooding in the Illawarra, Blue Mountains, Sydney Metropolitan and parts of Hunter and Central Coast districts on Monday.
The State Emergency Service responded to more early 400 requests for help overnight and performed about 120 flood rescues, with that number expected to increase.
Almost 70 evacuation orders are in place in Sydney's west, southwest and northwest for areas including the Hawkesbury-Nepean and parts of Emu Plains as well as Penrith in the foothills of the Blue Mountains.
There is major flooding at North Richmond, with river levels exceeding that reached in March (15.92 metres), with further rises possible on Monday morning.
SES spokesman Ashely Sullivan says even if the rain eases this week, as predicted, rivers will continue to rise because the ground is already saturated from the last flood emergency.
"We are seeing these rivers rise much faster than what's been predicted. Much faster than what we expected," he told the Nine Network on Monday.
"Things are happening quicker. The risk ... has increased significantly."
The SES is asking people to promptly heed evacuation orders and refrain from driving into flood waters.
"If you leave it too late, it becomes a rescue and our emergency service partners and the SES have to come and rescue you. We want to avoid that," Mr Sullivan said.
"We're still pulling people out of cars, we're attending a lot of properties where people left it too late."
Numerous evacuation centres have been set up across western Sydney.
"You will be looked after. Head to the evacuation centre. Play it safe," Mr Sullivan said.
A combination of heavy rainfall and rapid dam spillages is causing rivers to rise at an alarming rate, shattering previous records.
But the rain across the Sydney Metropolitan and Illawarra districts could start to ease during the afternoon and evening.
Six-hourly rainfall totals between 60mm to 100mm are possible.
Winds have gradually eased overnight after gale force warnings were issued on Sunday.
Emergency Services Minister Steph Cooke said on Sunday the Warragamba Dam is spilling at a rate of more than 500 gigalitres a day, which was more than during the floods in March and April this year that inundated large swathes of the state.
The SES was joined by 100 Australian Defence Force members on Sunday to help with sandbagging and doorknocking communities to warn of flood threats focusing on the Hawkesbury-Nepean area.
