A man is missing after a horrific jet boat crash on a West Coast river.

A local helicopter crew rushed to the scene and managed to save one man but they were unable to save the other.

Heath Bagnall and his crew from Ahaura Helicopters were conducting Department of Conservation spraying around the Taramakau River area on Friday afternoon.

His ground crew called him and told him the boat had crashed and two men were being washed down the river.

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Bagnall and his crew were able to pull one of the men out of the river but the couldn't save the other man.

Simon Lawn of Ahaura Helicopters said it had been a traumatic day for the men.

"They were pretty upset that they couldn't save both of them but I said they saved one person out there and that's the main thing."

It is understood the two in the boat were not wearing lifejackets or wetsuits.

A police spokeswoman said officers were called to the scene, about 1 km upstream from the William Stewart Bridge, about 2.25pm.

Police confirmed one person was rescued and treated by ambulance.

"We are still searching for one other person and the search will resume in the morning," the police spokeswoman said.

Emergency services will resume searching on Saturday morning.