A woman and young son have survived a terrifying ordeal after their vehicle went into the West Coast's Arahura River, north of Hokitika, overnight.

The boy, 6, escaped the vehicle and walked nearly two kilometres to State Highway 6, where he was found just before 2am by a member of the public, Sergeant Mark Kirwood, the officer in charge of West Coast Search and Rescue, said.

The woman's ordeal, which ended when she was found on a beach several hours later, was much longer.

After his own rescue the boy was able to tell police his mum's vehicle had crashed and she was in the water but he couldn't get her out, Kirwood said.

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A full scale and search and rescue operation began, he said.

"We put the call out for assistance and it was answered en masse by LandSAR volunteers, Fire and Emergency NZ, Surf Life Saving New Zealand, the Coastguard and members of the jet boating community.

"We were also greatly assisted by the new Garden City Helicopter which had a search member on board with thermal imaging equipment."

The 41-year-old woman was found about 7.30am on a beach seven kilometres north of the Arahura River, Kirwood said.

"It was established she had gone down the river and into the ocean before being washed ashore north of the river mouth.

"She is currently being treated for hypothermia in Greymouth Hospital."

The boy was unharmed and being cared for by relatives.

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West Coast Police thanked the large number of volunteers and partner agencies that helped in the search, Kirwood said.