Hundreds of residents in flood-ravaged Australia's southeast have been rescued by helicopter and boat after rivers rose rapidly, cutting off entire towns and inundating houses, highways, bridges and farms.

Emergency crews deployed more than a dozen helicopters to rescue trapped people from rooftops in the worst-hit towns in NSW. More than 200 flood rescue operations were conducted in the state over the 24 hours to Tuesday morning, while 24 emergency warnings remained.

LifeFlight, a medical transport service, said it rescued 18 people and 14 pets from the Central West region, including in the towns of Murga and Eugowra.

The LifeFlight helicopter flies over flooded homes. – Reuters/LifeFlight

A multi-agency search was under way for a man and a woman believed missing in Eugowra.

Rapid torrents of water reached roof height throughout the town early on Monday morning, destroying roads and infrastructure and taking many locals by surprise.

Dianne Smith, 60, was last heard from on Monday morning when she called a relative from her car in the historic town as it went under.

When unable to make contact with her, her relatives contacted police. 

Also believed to be missing is Ljubisa "Les" Vugec, 85, who was last seen at his home in Eugowra about 9am on Monday. 

Searches were taking place in the Eugowra area for the missing pair, involving numerous police agencies and the State Emergency Service. 

Premier Dominic Perrottet said wider rescue efforts were now focused on Eugowra and Forbes where 14 aircraft have been used to help stranded residents.

"We know for many of our communities in the central west, there is a lot of pain and a lot of hurt but we've got through these challenges in the past and we'll get through these challenges again," he said.

"We do expect a difficult period of time over the next couple of days."

An evacuation warning was issued for Forbes on Tuesday morning, saying the Lachlan River was rising faster than anticipated and about 1000 people in the town needed to leave immediately or risk being cut off.

With river levels rising faster than expected, about 500 to 600 homes and businesses are anticipated to be inundated again under the gushing water.

The LifeFlight helicopter flies over flooded homes. – Reuters/LifeFlight

On the 63rd day of the flood crisis, Emergency Services Minister Steph Cooke spoke of the ongoing devastation, saying Condobolin was entirely isolated.

"This just isn't people's houses. These are people's homes. It's their belongings, it's their memories, it's their livelihoods and it's their futures," she said.

About 100 Australian Defence Force personnel have been deployed to help rescue operations, with 12 New Zealand volunteers arriving, along with 14 aircraft supporting and rescuing residents and another four helping with logistics and transport.

SES Commissioner Carlene York described the response "as one of the biggest operations … across NSW in its history".

The Bureau of Meteorology's Steven Bernasconi said 17 flood warnings are in place with eight major alerts affecting 25 locations.

Major flooding was occurring at Bathurst where the river peaked at 6.4m but has fallen significantly and Eugowra, which peaked at 9m and is slowly falling.

Major flooding with an extended peak is also expected at Gunnedah and Wee Waa, west of Tamworth, from Tuesday into Wednesday. Both communities have been hit with repeated flooding in recent months.

NSW Premier Dominic Perrottet, NSW Minister for Emergency Services Steph Cooke and NSW SES Commissioner Carlene York look over current flood zone maps. – AAP

The evacuation order for Forbes came just over a week after it experienced record flooding that damaged homes, businesses and roads.

Forbes Mayor Phyllis Miller said people were shocked by another inundation.

"They can't believe this could happen twice in such a short period of time," she said. 

"This is really heartbreaking. Our farming community and businesses are suffering terribly."

The Wyangala dam is spilling at record levels, with 230,000 megalitres being released a day, exacerbating flooding along the Lachlan River.

Locals in Forbes look as floodwaters block the Newell Highway, which is one of the main routes through inland NSW. – AAP

Perrottet said his government was committed to raising the dam because it would protect lives.

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 said.

Wyangala dam overflows after heavy rainfalls. – Reuters