The Lachlan River is predicted to reach the major flood level at the Iron Bridge in town overnight, and the town is humming with emergency services and volunteers helping the community prepare.

SES vehicles laden with sandbags are barricading off businesses that could be impacted, and the sound of helicopters is almost constant overhead as they observe the progress of the water.

A weekend of rain on the back of a wet winter saw Wyangala Dam hit 104.9 per cent, with up to 80,000ML a day coming downstream on the weekend.

That is combining with water from downstream tributaries to result in a prediction of major flooding for Forbes, with the peak to arrive in town on Wednesday.

An evacuation order has been issued for low-lying areas, with more than 400 properties including residential, business and rural, to be advised to leave by 9pm Tuesday.

The water had closed roads to Cowra (Lachlan Valley Way), Eugowra (Escort Way), Bedgerabong (Bedgerabong Road) and Grenfell (Henry Lawson Way) by Tuesday afternoon, and the water was lapping at Lower Bathurst Street.

It's very much a case of "prepare for the worst, hope for the best" for some of the businesses who will be impacted if the water comes across Dowling Street as it has in earlier major floods.

The 2016 flood was notably different, and there is a real sense of "wait and see" when it comes to what this one does.

Russ McMahon's business is on the corner of Browne and Lawler streets, a couple of hundred metres from Lake Forbes.

He's sandbagged and lifted equipment, but is hopeful he won't actually see water in the premises.

"In 2016 the water's only come into the stormwater drains across the road ... but in 1990 it went a foot through the workshop, so we're allowing for that and hoping we only get 2016 floods again," he said.

"The old owners of the building told me ... as soon as the water starts coming from the lake (from the direction of Frogs Hollow) it comes in a hurry."

At the other end of the main street, Steve Lewis has seen a number of floods come through the Toyota premises where he's been for 35 years.

"Going back, 2016 it just got around us and it didn't get in," he said.

"2012 it went across the road but only by 100mm ... back to 1990 in August when we had the big flood, that was almost four foot inside the building.

"We've started to put things up off the ground at least anyway at this stage, and we'll keep an eye on it."

Across the street at Forbes pool, manager Damian Lehnen is prepared.

Between his efforts, Forbes Shire Council and swimming clubs, everything from pool covers and lane ropes to dry chemical and picnic tables has been removed from the site.

The pool closed at 11am Monday to begin the preparations.

Forbes Mayor Phyllis Miller said more than 400 homes had been doorknocked to advise them of the potential impact of flooding, and would be further advised of the evacuation notice.

"At the moment, we are hoping that everyone is taking care of themselves, taking care of their properties, knowing that the SES have sandbags," she said.

"Please take care, stay safe.

"Please don't drive into floodwater.

"Our roads ... we've had six weeks of rain, it might look okay but it could have a great big hole in it.

"Please take care of road closed signs and take care of yourselves."

Updated road closure advice at https://www.livetraffic.com/ or the Forbes Shire Council website for local roads.