An evacuation order has been issued for residents of Forbes in central-western New South Wales, but the State Emergency Service (SES) is concerned some people are choosing to leave their homes late.

Forbes is bracing for major flooding, with the Lachlan River expected to peak at 10.65 metres about midday on Sunday.

Flood warnings remain in place across the state's central west.

The evacuation order covers streets in the city's south, south-west, north, east and CBD.

Floodwaters have inundated properties on Ferry Street. ( ABC News: Luke Wong )

Residents have been advised not to delay their evacuation, with roads expected to be congested or closed.

SES said remaining in flooded areas could place lives at risk.

SES and Rural Fire Service volunteers have been doorknocking homes in parts of the town and urging about 1,000 residents to evacuate before 5:00pm on Saturday.

Michelle Mavroyeni from the SES said people choosing to evacuate later than that time could be putting themselves at risk.

"They may leave it to the night time, when floods and flooded roads are very difficult to see, or the flood conditions have changed where the evacuation route was once clear," she said.

"Since they last looked, it's then become blocked."

Evacuation orders are already in place for nearby Ungarrie and the Wagga Beach Caravan park.

Residents have been told they could be away from their homes for some time, and should pack for at least five days.

SES spokeswoman Becky Gollings urged residents to continue making preparations to leave their homes.

The SES has carried out three flood rescues overnight, and 90 flood rescues across the past three-and-a-half weeks since flooding began.

The Lachlan River at Forbes is expected to peak at 10.65 metres on Sunday. ( ABC News: Luke Wong )

Best thing is to leave early, SES says

SES acting commissioner Mark Morrow said some people had left Forbes already.

An evacuation centre has been set up, but residents were encouraged to stay with friends or relatives if possible.

Mr Morrow said the best thing was to leave early.

"If they do stay, there could be a loss of structure, they may lose sewerage pumps in town," he said.

Volunteer Stuart Jenkins helps fill sandbags at Forbes. ( ABC News: Luke Wong )

"They could lose power. There probably won't be telephones."

Mr Morrow said there were a lot of factors during flooding that could not be controlled, including dead livestock contaminating floodwaters, and chemicals that may leak into the water from farms.

"There is no good reason to stay if you are going to be inundated with water," he said.

There are about 1,000 people affected by the evacuation order.

The flood level is expected to peak at the Iron Bridge at Forbes. ( ABC News: Liv Casben )

'That's a lot of water going through Forbes'

Bureau of Meteorology senior hydrologist Hugh Bruist said the peak flood level at Forbes was likely to linger for up to 25 hours.

"It doesn't have to rise too much to cause a lot of damage there," Mr Bruist said.

"Yesterday (Friday) we were forecasting 10.6m, and today (Saturday) we're forecasting about 10.65m.

"That's not much, but when you consider just how wide and flat the floodplains are out there, that's a lot of water going through Forbes.

"Flood levels, we believe, will now be pretty close to a similar flood event which occurred back in August 1990."

Residents of Forbes get a closer look at the flooding. ( ABC News: Liv Casben )

School holiday traffic a concern

Families travelling for the school holidays are being urged to stay away from flooded areas in the region.

"The big concern is people during the school holiday break that might be intending on going through that area," Mr Morrow said.

"The Newell Highway is being impacted by this, which goes north to south, border to border.

"We're urging people to stay across on the eastern seaboard and make the most of the school holidays [there]."

Flood warnings are in place for nearby Condobolin, which is also affected by flooding.

"We are monitoring the Murrumbidgee River just in case there are river rises there through to Gundagai and through to north Wagga," Mr Morrow said.