A West Coast couple have spoken about the moment a mother who survived being swept down the Arahura River and out to sea after a car crash tumbled into their backyard.

At 7.15am on Saturday, Bronwyn Vercoe and her partner, John, saw a naked woman scrambling over a fence into their property.

Vercoe told Stuff she ran towards the woman, who looked "fearful", and gave her a towel to cover herself.

"I said, 'It's okay,' and put the towel around her. She was most worried about her child. She was saying, 'Where's my boy? I've lost my boy'."

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The woman told Vercoe she had "washed up here" after being swept into the Arahura River 7km away.

She allegedly said she had crashed her car through a fence, stumbled into a river, been swept away for "possibly hours" and washed ashore on a beach before she walked through gorse to the property.

One of the residents called the police, who had been searching for the woman all night, since her 6-old-year son raised the alarm with a passing motorist around 2am.

The child walked nearly 2km to the main road, State Highway 6, after he couldn't get his mother out of the crashed car.

The woman told Vercoe she had been swept into the Arahura River – 7km south of the Awatuna property – and "washed up here".

She'd been "caught in the undertow, but managed to swim down and pop back up out of it", Stuff reported.

Sergeant Mark Kirkwood, who oversaw the search and spoke to the woman afterwards, called her survival "miraculous".

"She is a lady who has a great will to live."

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Kirkwood said the past two rescues from that river had ended in a body recovery.

"You go into a West Coast river, usually it's not going to end well," he said.

Police called on everyone able and willing to search through the night, with searchers coming from local speed boating and surf clubs, joining police, and from the Lake Brunner Coastguard.

"We searched the area, we went along the beach, searched the river. We had people on quad bikes, on foot, in jet boats, about 40 people thereabouts. People heeded the call in the middle of the night, from Westport, from all over."

Vercoe, who said the woman's survival was a "miracle", made her Milos to drink while they awaited emergency services.

"She told me she could see the helicopters searching for her, but they were in the wrong place," she said.

"It was so confusing. Such a miracle. Far out. It wasn't her time."

The woman was treated for hypothermia in Greymouth Hospital, while her son remained in the care of relatives.

Kirkwood said inquiries into the event were ongoing.