On Monday afternoon, the flooding at Forbes was rated as “minor” with water levels at 9.3 metres. The bureau said this could reach a major level of 10.65 metres during Wednesday.

“We’re obviously concerned that the flood levels at Forbes may reach levels as high as 2016, and even possibly higher,” Justin Robinson, from the bureau’s national flood operations, said.

He said floodwaters were taking a “little bit longer” to get to Forbes than as per previous warnings.

“Even though the rainfall has stopped, the flood is still in motion,” he said.

“Now we’re expecting the floodwaters to exceed that major flood level overnight [on] Tuesday and Wednesday. We might see that peak actually occur sometime during the day on Wednesday.”

The SES is advising Forbes residents to “prepare to evacuate” by Tuesday.

Chief Superintendent Swindells said that, pending guidance from the bureau, that evacuation warning could be updated to an evacuation order for up to 800 properties and issued as early as Monday afternoon or evening ahead of Wednesday peaks.

SES crews door-knocked those in at-risk properties on Sunday to warn them.

“If you are in Forbes, your best opportunity is to start preparing your home or business and make arrangements to move to safety before flooding starts affecting you,” the SES said.

“The situation is likely to change, and it can escalate quickly.”

Forbes Shire Council mayor Phyllis Miller said it was a positive sign that only 40,000 megalitres of water was being released from Wyangala Dam on Monday, down from 100,000 megalitres in recent days.

“We’re a flood town, we’re used to it,” she said.

“You prepare the very best you can and hope for the best.

“We’re hoping we don’t get any further rain that will cause us grief.”

The Lachlan River reached moderate flood levels at Cowra, where the water level has started falling.

The river at Nanami is likely to peak near 12.4 metres on Tuesday morning with major flooding, slightly higher than the 2016 peak, as floodwaters from Cowra combine with those from the Belubula River.

Bureau meteorologist Sarah Scully said a “large amount of water” was making its way downstream towards Forbes, and major flooding was also expected about Cottons Weir and Jemalong from Wednesday.

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She said, due to the large catchment size, the flooding was slow moving and might take a couple of weeks to subside.

“The good news is no significant rainfall is on the forecast across the region over the next few days,” she said.

“However, there is the potential for showers and storms to return across the area on the weekend.”

The Forbes Olympic Pool and Apex caravan park have closed indefinitely until the threat has passed, the mayor said.

A walk-in evacuation centre has been established at the Presbyterian Church on Harold Street. Forbes Town Hall, Concourse and Council Chambers will house the Emergency Operations Centre during the flood emergency.

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