Rescue helicopters, military personnel and international emergency crews are working across NSW flood zones as people in the central-western town of Forbes brace for their second inundation in a fortnight.
More than 160 emergency personnel, including 12 volunteers from New Zealand, have been deployed to help in the flood-ravaged central-west.
As decimated communities clean up, emergency services are searching for two people missing in the town of Eugowra, which was devastated by roof-high flash flooding early on Monday.
Dianne Smith, 60, last spoke to a relative on the phone from her car on Monday morning and Ljubisa "Les" Vugec, 85, was last seen at his Eugowra home around the same time.
State Emergency Service chaplain Steve Hall said Eugowra has been decimated in the disaster.
"Everything they hold dear has been swept away in a wall of water. It's a long and hard road (to come back), but these people will need to dig deep," he said.
The Bureau of Meteorology is warning the Lachlan River at Forbes could meet the historic June 1952 peak of 10.8 metres by Wednesday morning, while major flooding is expected to persist until the end of the week.
People in parts of the north-western town of Gunnedah have been told to evacuate as the area is hit with major flooding.
The Namoi River is predicted to peak near 8.2m early in the day.
In the 24 hours until Wednesday morning, the SES responded to 329 calls for help and performed 17 flood rescues.
Fourteen people were rescued in Forbes at around 8pm on Tuesday night after the Plainsman Motel was inundated.
A rescue helicopter rescued an elderly couple, their daughter and two small dogs from a Forbes home that was threatened by rising waters about 10pm.
Just before dawn on Wednesday, two people were plucked from the roof of a submerged vehicle in flood water at Condobolin.
The SES has issued 25 emergency warnings across the state, while in Forbes about 1000 people and up to 600 homes and businesses are affected by evacuation orders.
Crews have been door-knocking and sandbagging after 120mm of rain fell in a matter of hours early on Monday, causing ferocious flash flooding and forcing Wyangala Dam to spill into the swollen Lachlan River.
Fourteen aircraft have been designated to support stranded residents and another four are helping with logistics and transport.
SES Commissioner Carlene York said the response was one of the biggest operations in NSW in the state's history.
The torrential rain doubled the height of Mandagery Creek at Eugowra, east of Forbes, early on Monday, devastating the town of 800 people.
Kelly Chambers was celebrating her twin daughters' 23rd birthday on Sunday night in the house the family bought less than a year ago.
Just hours later, they climbed out a window and waded through waist-deep water as a torrent tore through the village.
Ms Chambers, her husband, their three children and her parents eventually found a truck trailer to perch on among a group of about 20 people and seven dogs.
They stayed there for six hours before a fire truck delivered them to the evacuation centre at the showground, from where they were flown to Orange for the night.
"It's devastating. Anything that is a memory is gone," Ms Chambers told AAP on Tuesday.
Eugowra residents have described two sudden and intense surges of water flowing through the town, washing away houses, knocking over structures and leaving destruction like a "war zone".
Clean-up operations have begun at towns and villages including Canowindra, Cowra and Cudal, which were also hit by flash flooding, leaving homes and shops in muddied ruin.
The SES is planning to deploy more international teams, with storms predicted in the state's south on the weekend.
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