Major flooding forecast in northern NSW

© iStockphoto.com/Carmen Martínez Banús

MAJOR flooding is expected across northern NSW in coming days, leading to predictions of scenes not witnessed since the great floods of 1990.

An evacuation order was issued for the Weilmoringle and Goodooga communities, in Brewarrina Shire, on Friday.

The State Emergency Service (SES) has warned residents to leave the area no later than Tuesday due to the risk of property damage.

An SES spokeswoman told AAP on Monday that residents were in the process of leaving their homes, with most organising alternative accommodation with family and friends.

It comes after heavy rain deluged southern parts of Queensland, causing the biggest floods in 100 years, some of which is now flowing across the border into NSW.

SES operations controller Tony Casey predicts the Weilmoringle and Goodooga flooding will cause "significant damage" within the week.

"The forecast flooding is likely to cause significant damage to homes and will pose a direct threat to the residents if they wait too long before they evacuate," he said in a statement.

The Bureau of Meteorology says major flooding is also expected on the Culgoa, Bokhara, Birrie, Paroo, Barwon and Narran river systems, all in the state's north.

Flood levels are expected to exceed those of April 1990, when vast areas of land in northern NSW and southern Queensland were submerged and hundreds of homes damaged, the bureau added.

Other evacuation orders will be made if or when they are necessary, the SES says.

River levels in the north of the state are already high following heavy rain since Christmas.

They are now being increased further by heavy rain in Queensland, which is filtering into NSW.