Severe weather threatening inland NSW marks "the start of a very long season" as the state faces a third consecutive La Nina, Emergency Services Minister Steph Cooke says.
Extensive flooding and heavy downpours have hit multiple areas in the far west, as the system bearing the inundation churns east towards the already soaked Northern Rivers, Mid North Coast and Hunter regions.
"We're facing challenges on multiple fronts with prolonged flooding through the west and northwest," Ms Cooke said on Thursday.
Flood-weary communities faced days of uncertainty as the dangerous conditions tracked coastward, she said.
"We're at the start of a very long season, we expect.
"This is the third consecutive La Nina that has been declared by the Bureau of Meteorology and as we have seen all year, our catchments, our landscapes are highly saturated."
The cloud band that dumped rain across large parts of inland NSW this week was settling across the state's north coast, Bureau of Meteorology Hazard Preparedness and Response Manager Jane Golding said.
Parts of the north had copped up to 100mm of rain in the past 24 to 36 hours and further falls and thunderstorms were likely on Friday and Saturday, she said.
Some of the north coast would see totals of more than 100mm and isolated "bullseye" areas even more.
"Where those bullseyes are in particular, that's going to be where we see some significant flash flooding issues," she said
A number of inland river catchments are in flood and this was expected to be the case for the next couple of months, Ms Golding said.
Multiple climate drivers were behind the scenario including the La Nina system and a negative Indian Ocean Dipole and positive Southern Annular Mode - which describe wind and ocean current around Australia.
"It's very similar to what we saw last year," she said.
A flood watch extends from the Queensland border to Wollombi Brook in the Hunter, taking in the Central Coast and the Tweed, Brunswick, Richmond, Wilsons, Orara, Nambucca, Macleay and lower Hunter rivers.
In Lismore, moderate flooding is possible on the Wilsons River and further north on the Tweed, while in Grafton, Taree and Coffs Harbour, heavy rain throughout Thursday and into Friday could spark flash flooding.
Inland rivers already experiencing major flooding including the Macquarie, Lachlan and Namoi were hit with between 25 to 50mm of rain overnight.
The SES warned residents and people travelling around the state to monitor road conditions and avoid unnecessary trips.
Its volunteers had carried out 49 rescues and taken more than 500 calls for assistance, NSW commissioner Carlene York said.
She said the latest system moving into already flood-ravaged communities was of concern and preparations were ongoing.
A police officer on Thursday morning rescued a four-year-old boy and two women using a rope, after they became stranded in their car west of Dungog.
Multiple rescues were also undertaken in Warren, west of Dubbo, where major flooding on the Macquarie River continues, including people retrieved from submerged vehicles.
The town is protected with a levee although it has deteriorated in recent years, and the local council was awarded a $5.3 million federal grant to begin rehabilitation works in May.