A mother and her young son and daughter are believed to have drowned after their car plunged into a flood swollen in a northern NSW river this afternoon.

Police have located the submerged car near the northern bank of the Tweed River but are yet to recover the trio's bodies.

The alarm was raised when a 10-year-old girl managed to escape and was seen hysterically screaming for help.

According to Seven News, witnesses said the young girl was screaming that her mother, brother and sister were all trapped in the car.

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The vehicle plunged into the river after running off Dulguigan Road at Tumbulgum about 1.40pm.

NSW Police said sonar equipment had located the car submerged about five metres from the river bank about 3.25pm.

The bodies of the mother and her two children are believed to still be inside.

What caused the car to plunge into the river is not yet known.

Nearby residents frantically tried to rescue the occupants of the car but told reporters at the scene, they could see only bubbles rising to the surface.

Retired highway patrol officer Matt Grinham was passing by when he came across the scene.

He told the Gold Coast Bulletin that he dived into the water and followed the bubbles from the submerged car as it was washed downstream.

"We were going down feet first trying to work out where the car was," Mr Grinham told the publication.

"Then we tried diving a couple of times but the water was so brown and cold ... the flood run off is freezing down there.

"The car was completely submerged when we got there."

Another witness, Peter Taylor said the girl's feet were bleeding as she ran up the road desperately seeking help.

"This little girl has run down the road, and she has said her mum and dad or family have gone into the river," he told Nine News.

"She is only a little thing and her feet were bleeding apparently, that's what the fireys tell me, and they called an ambulance and called the police."

The girl has been treated in hospital, where she remains, being cared for by family members.

Northern Region Assistant Commissioner Jeff Loy described the crash as a tragedy.

"This is a tragic event and we're very concerned a tragic event will unfold over the evening," he said.

"This is about heeding the warnings of the road closures, this has been a major event and the emergency services don't take those closures lightly."

Local road closures are in place near the crash site, as a large emergency services contingent remains on scene.