Hundreds of people in the NSW central western town of Forbes have been ordered to evacuate before major flooding inundates the district within hours.

The evacuation order was issued by the State Emergency Service about 11am on Tuesday, warning 1800 residents to leave before 9.30pm.

SES Commissioner Carlene York told reporters 800 homes may be inundated, with the Lachlan River expected to match or exceed major floods in 2016.

The flood has been slow-moving and is not expected to peak until midnight, but she urged residents to get out early.

“It just adds additional risk, people trying to get out at night and trying to get to safety, because they can’t judge the water or they can’t see what roads have isolated them,” she said.

Already the SES has conducted more than 35 flood rescues – many unnecessary – putting the lives of volunteers at risk.

And more are inevitable, with volunteers reporting some people are saying they’ll refuse to leave their homes.

Ms York pleaded with them to change their minds.

“It’s not unusual for people who have lived in that area and lived through a number of floods to say, it didn’t flood last time, I’ll take the risk and stay,” she said. 

“But every flooding is different and water flows in different ways.  

“I would rather someone move out of their home for a day or two and come back and find everything in place than having to rescue them if the water’s coming up through their home.”

An evacuation centre has been set up at St Andrews Presbyterian Church for those unable to get to alternative accommodation with family or friends.

SES and Rural Fire Service volunteers are doorknocking and delivering about 10,000 sandbags to residents as well as helping prepare properties.

The Lachlan River is expected to peak at 10.65 metres in Forbes on Wednesday.

Moderate flooding has already occurred upstream at Cowra, and major flooding at Nanami, where the river peaked at more than 12 metres.

Downstream from Forbes, major flooding is also expected at Cottons Weir and Jemalong from Thursday.

The Macquarie, Paroo, Macintyre, Belubula, and Snowy rivers have also flooded over recent days, after parts of the state copped a month’s worth of rain in days.

“Some of those locations (received) even two to three times what you would get in November,” meteorologist Agata Imielska said on Monday.

“Catchments are wet, dams are full. This is exactly what we’ve been talking about for this season, with the elevated flood risk.

“It is a really important time to really be keeping a very close eye on any flood warnings.”

AAP

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