The State Emergency Service issued the evacuation warning, saying the Lachlan River is rising faster than anticipated and people need to get out by 7am on Tuesday or risk being cut off.
There were 120 warnings in place across the state, including 24 at emergency level and more than 12 evacuation centres have been established.
The Bureau of Meteorology said major flooding would occur at Forbes from early on Tuesday and water levels would peak around 10.8 metres on Wednesday - similar to the level reached in the June 1952 flood.
The warning comes just over a week after Forbes experienced record flooding that damaged homes, businesses and roads.
Forbes Mayor Phyllis Miller says people are shocked by another inundation so soon.
"They can't believe this could happen twice in such a short period of time," she told ABC TV on Tuesday.
People were working hard sandbagging to protect their properties but needed more support.
"This is really heartbreaking. Our farming community and businesses are suffering terribly."
The Newell highway was closed, with no hope of through traffic for some time.
"It's terrible, really, for the economy of a small rural town," she said.
Forecasters also warned of significant flooding in Bathurst, where possible record flood levels of more than seven metres would see flood levees breached.
People in the southern border town of South Albury and those at Western Plains Tourist Park in the central west have also been ordered to evacuate.
Moderate to heavy rain across the Lachlan River catchment has resulted in renewed river rises and major flooding at Cowra and Nanami.
The Wyangala dam is spilling at record levels, with 230,000 megalitres being released a day, exacerbating flooding along the Lachlan River.
About 150 people were rescued on Monday from roofs in the central-west town of Eugowra and nearby Molong, with extensive damage in the historic town.
The SES says 160 emergency service personnel were deployed to the area, with 14 helicopters supporting flood rescues and another four helping with logistics and transport.
SES Chief Superintendent Dallas Burnes said people in Eugowra were shocked at how quickly the water level had risen.
"The velocity was extremely fast - too fast in many cases to put boats in the water - hence the evacuations we performed yesterday with the 12 assets we had on hand via helicopter," he told Nine's Today Show.
The SES had performed 222 flood rescues and received 909 requests for help in the 24 hours to Tuesday morning.
A cold front will move to the south on Tuesday and snow is expected in southern parts of NSW before a high-pressure system from the south leads to conditions clearing on Wednesday.
Emergency Services Minister Steph Cooke says the situation is very serious but the emergency crews are doing a "magnificent job in the field".
"At times we had up to 14 aircraft in the air, rescuing people off rooftops," she told Seven's Sunrise.
"We are in very challenging times. We are in our third consecutive La Nina event.
"Our communities are strong. They are resilient. We will keep doing this day in, day out until we are through it."
Four people had died in the past 62 days as a result of the severe weather, although no deaths were reported in the previous 24 hours, she said.
Twelve flood rescue operators from New Zealand arrived on Tuesday to support exhausted SES crews in Parkes.
Ms Cooke said the SES was also working with authorities in Singapore and the United States to secure additional support.
Many local roads are closed and in the northwest.
Collarenebri, Lightning Ridge, Tilpa and Weilmoringle communities are isolated, while Brewarrina can be accessed by trucks only.
The SES says a regular resupply of goods and services has been instigated to service Walgett, Collarenebri, Wee Waa, Lightning Ridge, and Carinda.
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