The Lachlan River at Forbes in the state's central west reached a peak of around 10.7 metres on Saturday, slightly below a record set in 1952.
Floodwaters in the town, which has been effectively cut in two due to road closures, were receding on Sunday morning although it remained too early to measure the impact of the inundation.
State Emergency Service chief superintendent Ashley Sullivan said the town had experienced major flooding causing a significant impact on the community.
"It's going to be a big couple of days of emergency response in this community until these floodwaters recede and then we can get in and do those damage assessments," he told ABC TV.
SES Forbes unit commander Robert "Rocky" Walshaw said the clean-up would be a slow process as it took a long time for the floodwaters to drain away.
"I've seen shots of it from the air, from the (helicopters), and it's just water everywhere," he told AAP.
Emergency responders in Forbes have warned conditions in the town are hazardous due to fast-flowing water through its centre and the risk of contaminants such as toxic chemicals and sewerage.
In Wagga Wagga, evacuation orders were lifted and people were told they could return to their homes after the Murrumbidgee River peaked on Friday at 9.72 metres, the highest level in a decade.
The NSW SES fielded 105 calls for help in Forbes on Saturday, while statewide it received nearly 280 requests and carried out 27 flood rescues.
There were 10 major flood warnings in place across NSW on Sunday morning, including for potential inundation at Narrandera on Tuesday.
Renewed rises were likely at Carrathool and further downstream at Hay, with major flooding at the latter possible from mid-November.
Across the border in Victoria, more than 40 flood alerts remained on Sunday morning as authorities warned people at Kenley on the Murray River in the rural city of Swan Hill to evacuate immediately.
Water levels at Mildura are expected to continue rising throughout November before a possible high in December.
Along the Murray River at Echuca and Moama, major flooding is expected to continue during the week although water levels are falling.
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