A three-year-old boy has drowned in a stormwater drain and several people have been rescued from floodwaters which have cut off properties in northern New South Wales.

The boy was reported missing about 12:30pm (AEDT) at Bingara, near Inverell, and emergency services found his body half-an-hour later in a nearby stormwater drain that had swelled with water from the recent heavy rain over the town.

The NSW State Emergency Service says there is major flooding around Moree and Graves End as the Gwydir and Namoi River catchments swell, and are warning floodwaters could linger for several weeks.

Fourteen people around New South Wales have needed to be rescued from floodwaters over the last two days, mostly around Sydney and the state's north.

Four people were rescued from cars that became trapped in flooded causeways near Boggabri and Narrabri this morning.

Phil Campbell from the SES says a helicopter had to be used to rescue two people from a bogged tractor near Moree.

He says the SES expects numerous properties to be cut off for days.

"This major flooding is going to lead to the isolation of numerous properties around Moree, and there will be an extended period of resupply which will be required in coming days," he said.

The town's residents are now preparing to evacuate, with the weather bureau predicting the Gwydir River will peak at 9.5 metres tomorrow night.

Major flood warnings also remain in place for Wee Waa on the Namoi River, with moderate flooding also expected at Tamworth on the Peel River.

The floodwaters have closed the Newell Highway between Moree and the Queensland border, and the Gwydir Highway is also closed for 20 kilometres east of Moree.

Drivers are being warned to avoid the area.

Mr Campbell says the worst of the rain is expected to begin clearing this afternoon, but the flooding will not ease for some time.

"We are seeing areas downstream of Ashford such as Yetman that are likely to experience flooding in coming days," he said.

"There's also the expectation that flooding will develop on lower parts of the Namoi River, downstream of Gunnedah, particularly around Wee Waa over the course of the weekend.

"The nature of the flooding, particularly in the north-west, means that floodwaters are likely to persist for some days and in some areas possibly for several weeks."

Police are reminding people not to drive, ride or walk through floodwater and to listen to their local radio station for updates and advice.

ABC/AAP