The Lachlan River is expected to reach a major flood peak of 10.6 metres later on Thursday or early on Friday.
The State Emergency Service has ordered people to leave Forbes before 8pm on Thursday.
The orders affect about 250 homes as rain batters the region and moderate flooding affects Wagga Wagga after the Murrumbidgee River peaked on Thursday morning.
The Bureau of Meteorology says southern inland NSW will be hit with widespread heavy rain and flash flooding as damaging winds continue on Thursday.
"Widespread rain, combined with wet or already flooded catchments, will lead to renewed river and creek rises," the bureau said in a statement.
"A number of flood warnings have already been issued and flood watches are in place as rivers begin to respond to rainfall."
Heavy rain with six-hourly totals of 30 to 50mm will extend across southern NSW, with damaging winds in alpine areas.
Rain will ease on Thursday night, with showers moving across the coast on Friday morning before finally moving offshore in the afternoon and evening.
But the fortnight ahead will be soggy with above median rainfall (greater than 80 per cent chance) on the cards for large parts of the eastern two-thirds of Australia.
Meanwhile, a search is under way for a man believed missing in western NSW floodwaters.
Police say the 63-year-old was last seen on a rural property on Lachlan Valley Way at Hillston around midday on Tuesday, when he told a friend he would return shortly.
Several hours later they were notified he was missing and began searching.
Local police, PolAir, Police Rescue, the SES and paramedics failed to find any trace of the man on Wednesday and the search is now focused on the Riverina farm and along the Lachlan River.
Fears for his safety come after police found another man's body trapped in his submerged ute in the state's central west on Tuesday.
Catchments are very wet and most dams are at or near capacity.
Senior forecaster Jonathan How said on Thursday that Broken Hill in the far west of NSW had copped an overnight deluge.
"They've now set a new record for the highest October daily rainfall record - more than 50mm overnight," he told ABC TV.
Sunny skies should provide a brief weekend reprieve but many places will remain flooded.
In Forbes, SES volunteers have been filling sandbags for residents.
Local commander Rocky Walshaw said on Wednesday it was hard to predict how bad the flooding would be with creeks already swollen.
"Every flood is different but this one is going to be here for a long time," he told AAP.
"This peak is going to stay here for five days but when the rain falls again, it's about where that's going to fall."
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