"This is a very serious situation," Emergency Services Minister Steph Cooke said on Monday.
"The entire central west of NSW has been hit very hard in the last 24 hours.
"The ground is so saturated, that all it takes is one heavy downpour to cause the type of life-threatening flash flooding we have experienced overnight and into today."
The main areas of severe impact are the towns of Parkes, Forbes, Molong, Eugowra, Cowra, Canowindra, Blayney, Young and Yass.
Some 140 flood rescues were carried out in the small town of Eugowra, including 100 from roofs.
"With a population of 700, one in five residents have been rescued in the last few hours, by helicopter or by boat," Ms Cooke said.
"This (severe weather) event is now in its 62nd day, and every day it throws up new challenges."
Four people died in the last 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 were flying in from New Zealand to support exhausted State Emergency Service crews in Parkes, Ms Cooke said.
"The SES is now also working with authorities in Singapore and the United States of America to secure additional support."
It was the first time the SES had made a request for overseas assistance, SES Commissioner Carlene York said.
"Because of the length and the devastating effects of these floods."
The SES had responded to 173 flood rescues and 470 requests for help since 5am on Monday.
In the small central west town of Molong, dozens of residents sheltered in the local RSL overnight, as fast-moving floodwaters rose to roof height, and Australian Defence Force personnel rescued people from roofs.
"It was just unbelievable how fast the water came," Mary Mulhall told AAP from the Molong RSL Club on Monday.
"It rose really quick and then it sort of went down fairly quickly as well but it just devastated on its way through."
About three-quarters of the historic town is damaged, with numerous roads ripped up and the new hockey field destroyed.
"The shop owners would be devastated because most of the shops have all gone and (the) supermarket's just gone altogether."
Waters are beginning to recede in Molong's main street, while SES and Fire and Rescue crews begin a massive clean up.
It comes after both agencies launched into action about midnight, doorknocking homes, beginning evacuations and helping the ADF rescue people stranded on roofs.
Fire crews also rescued two people and their dog from knee-deep water inside a house, another couple from the top floor of a motel and waded through chest-high water with a police officer to reach another woman stranded in her inundated home.
At least two large shipping containers were carried through the centre of the historic town by floodwaters and are blocking the Mitchell Highway.
Premier Dominic Perrottet urged people in flood-hit areas to follow emergency warnings.
"It may be the case that you don't see flooding around you when those orders are put in place - that is because we expect and predict further flooding moving forward," he said.
He urged people not to drive through floodwaters, saying a significant number of rescues were because people tried driving on flooded roads.
"You wouldn't drive through a bushfire - don't drive in floodwaters."
Some 116 warnings have been issued by the SES, including for residents to evacuate to higher ground in Cowra, Canowindra, Derrinwong and Eugowra.
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