Major flooding is occurring on the Macquarie, Darling, Culgoa and Lachlan rivers, a risk of inundation is current for multiple inland catchments and forecasters are concerned about the possibility of landslides.
The scenario has prompted authorities to activate Sydney's State Emergency Operations Centre. Experts will monitor the situation in real time and co-ordinate a whole-of-government response from the hub.
The Bureau of Meteorology's Helen Kirkup says a trough tracking towards the coast will develop throughout Saturday and likely bring intense rain and possible flash flooding.
Six hourly totals of 30-50 millimetres are predicted for the Central Tablelands and Hunter regions as well as metropolitan Sydney, with isolated areas expected to cop up to 70mm.
Damaging winds are also likely across Sydney and along the Illawarra coast into Sunday, with peak gusts of more than 90km/h.
On top of a soggy week, heavy rain on already saturated grounds would heighten the risk of landslides and debris falling across roads, Ms Kirkup said.
The system is expected to blanket the state throughout the weekend, with widespread showers forecast from top to bottom.
Supercars enthusiasts camping at the Bathurst 1000 have been warned of stormy and potentially dangerous conditions, with heavy dumps threatening at Mount Panorama on Saturday and Sunday.
Up to 100mm could drench the tens of thousands of fans attending and potentially trigger flooding along the Macquarie River in Bathurst.
Free sandbags are available in the town, while volunteers are checking in with residents and tourists camping on the river's banks.
As flood peaks move down already swollen rivers around the state, the BoM has warned of renewed risks for the Gwydir, Namoi, Macquarie and Belubula rivers, Mandagery Creek, the Lachlan and Bogan rivers, Colo River, Wollombi Brook and Lower Hunter River.
The swelling Hawkesbury River and consistent rain across the Hawkesbury-Nepean Valley have prompted alerts for people from Lower Portland to Windsor to stay informed of the situation.
SES Assistant Commissioner Dean Storey said on Saturday the focus would be on potential flash flooding in Greater Sydney, the Hunter and Illawarra in the next 24 hours.
"Particularly with so many people out on the roads and given it's the weekend but also a lot of people are travelling home with school holidays, just really urging those people to drive to the conditions - plan your trip accordingly," he told the ABC.
"If you are in those affected areas or travelling through those affected areas, monitor those conditions and never drive through floodwaters."
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