Floods triggered by heavy rains have isolated two towns in northern New South Wales and swept away a boy leading to his death.

The State Emergency Service (SES) have sent four helicopters and a plane to drop food, water and medicines for some 1,800 residents stranded in Wee Waa and for about 200 other people in Moree on Sunday.

The SES also ordered the evacuation of 30 homes in Moree as the nearby Meehi River is seen to overflow. Evacuees can be accommodated at the Moree town hall.

"I've just flown over the area in the chopper and it's very extensive, the amount of flooding," regional emergency controller Steve Martin said, according to ABC Radio. "The cross-country water that's going there... it's created an inland sea and it's quite spectacular when you're in the air, but not so happy for the people on the ground."

Meanwhile, the Newell Highway between Moree and the Queensland border is impassable until Monday.

The same floods drowned a three-year-old boy on Saturday when he was swept into a stormwater drain at Bingara, according to Sky News.

The floods damaged crops dealing wheat farmers a cruel blow, said Narrabri mayor Robyn Faber.

Wee Waa is expected to be isolated until Thursday or Friday as the weather is forecast to bring more rains to northeastern Australia on Wednesday or Thursday.

The water level of the Meehi river is expected to breach the 10 meter limit by Sunday. Flood warnings were also placed for the Macintyre, Macleay and Bellinger rivers.

An SES helicopter used for supply dropoffs and rescue.