The NSW State Emergency Service had performed six rescues and responded to 330 calls for help over a 24 hour period as of Saturday afternoon.
Towns and properties were cut off by flash flooding and roads across the state were closed due to the conditions.
The Bureau of Meteorology's Jane Golding said the rain would intensify throughout Saturday into early Sunday east of the Great Dividing Range, with the storm system creating the downpour travelling from the state's west to the coast.
It will begin easing by midday, with some sunshine forecast for the start of the week.
"The good news is this system is moving through pretty quickly and we should have a couple of days of reprieve," Ms Golding said.
NSW Premier Dominic Perrottet urged people to stay safe, be cautious on the roads and avoid driving through flood waters.
"Don't put your life at risk, your family at risk or our volunteers at risk," he said.
There are 63 flood warnings in place and residents across NSW, including in Sydney, were told to stay informed of local conditions.
Four defence helicopters were to be on standby from 6pm on Saturday in response to a request for extra help from state authorities.
Parts of NSW saw up to 100 millimetres of rain by Saturday evening, and full dams and saturated catchments left rivers prone to flash flooding.
Major flooding is occurring on the Macquarie, Darling, Culgoa and Lachlan rivers, there is a risk of inundation for multiple inland catchments and forecasters are concerned about the possibility of landslides.
Damaging winds are also likely across Sydney and along the Illawarra coast into Sunday, with peak gusts of more than 90km/h.
Supercars enthusiasts camping at the Bathurst 1000 have been warned of stormy and potentially dangerous conditions over the weekend.
Organisers cancelled one of Saturday's racing events with the wild weather causing chaos for drivers on the Mount Panorama track.
Sunday's race is still scheduled to go ahead.
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.
Mr Perrottet urged attendees to look out for each other when they headed home on Sunday, with the end of the event likely to see packed roads.
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, the Colo River, Wollombi Brook and the Lower Hunter River.
NSW SES Commissioner Carlene York urged people to remain vigilant throughout the coming week.
"It is important to remember that the rains will pass and the sun will come out, but the rivers may still be rising," she said.
Emergency Services Minister Steph Cooke said NSW was only eight days into its storm season and flooding was impacting rural, regional and remote communities.
"It is expected to do so for weeks and in fact months ahead," Ms Cooke said.
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