The State Emergency Service (SES) has warned conditions in the Hunter Valley "could get a bit scary" if the river reaches an expected record level.
An evacuation warning remains in place for the Singleton township, due to major flooding.
SES controller David Monk said the river level in Bulga was expected to peak at 9.1 metres on Wednesday night, the highest level since 1952.
"As a result of that we're looking at around about 13.8 metres tonight or this afternoon at Singleton which will certainly see a number of road closures up there, with conditions starting to get a little bit scary," he said.
Evacuation orders are still place for several other towns, including Bulga and Broke.
Although rain has eased in Sydney, there are heavy falls predicted further north particularly between Bellingen, Nowendoc and Foster.
There is a possibility of six-hourly totals in excess of 200 millimetres, according to the Bureau of Meteorology (BOM).
Premier Dominic Perrottet said there was a "significant risk" of flash flooding at Singleton, where the Hunter River could peak at 13.8 metres.
"As I have said, this event is far from over," he said.
Electricity provider Ausgrid is warning residents that power may have to be cut off around Singleton soon to protect important electrical infrastructure.
Flood Recovery Minister Steph Cook said more than 500 people had arrived at recovery centres opened in the past 24 hours at Cessnock, The Entrance and Singleton.
Broke, in the Hunter Valley, is experiencing its biggest flood on record.
Local winemaker Andrew Margan said most of his vineyard was underwater.
"No power, no water, we can't get out, we are surrounded on all sides by water ... it's pretty devastating," he said.
"I've never seen this before ... the river has just come over the top of the vineyard and it's never done that."
Other villages in the area, including Wollombi and Bulga, have been cut off by floodwaters and SES rescue boats are being used to evacuate residents.
Dozens of evacuation orders are still active in parts of Sydney, with major flooding still occurring along the Hawkesbury and Colo rivers.
However, the SES said the Hawkesbury river was slowly easing at North Richmond, Windsor, Sackville and Lower Portland.
On the Central Coast, major flooding is also impacting Tuggerah Lake due to significant river level rises along the Wyong River.
The low pressure system is expected to quickly shift off the east coast later this evening with drier and more stable air developing in its wake.
The SES has received 1,200 calls for help and have carried out 21 flood rescues since 8pm yesterday.
Emergency service volunteers are assessing the damage of flooded homes today in several areas, including Greater Sydney, NSW SES Commissioner Carlene York said.
"Our engineers have to get in to make sure that those bridges are safe ... [we're] also expecting significant damage to the roads," Ms York said.
She said people from flood-affected areas need to wait for the all-clear before they can return home.
Some residents have already been told they can return with caution, including those who live in parts of Georges Hall, Lansvale, Moorebank and Chipping Norton in south-west Sydney.
From tomorrow the low pressure system will move out to sea and Ms Golding said Friday could be “quite a nice day” along the coast.
"But it will take a bit of time for the floodwaters to come down," she said.
