A helicopter pilot who daringly tried to prevent tragedy has spoken of the moment he knew he could only help one of the two men washed away in a swollen West Coast river.

Two men were washed downstream on the Taramakau River after a jet boat crash yesterday but only one was rescued by a helicopter that happened to be in the vicinity.

Ahaura Helicopters pilot Heath Bagnall was spraying around the area for the Department of Conservation when ground crew called and alerted him to the crash.

"[They said] 'They might need help', and then he sounded a bit more assertive on the phone - 'definitely come up, one is starting to go under'," he recalled.

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Bagnall jettisoned the chemicals onboard the Robertson helicopter to make room and grabbed a crewman to help.

The pair flew first towards the man they believed was in greater distress and threw empty fuel canisters into water, hoping to give him something buoyant to hold on to.

Four jet boats with Surf Rescue personnel on board, and four ground crews are searching the area between the Taramakau bridge and Kumara-Inchbonnie Rd. Image / Google Maps
Four jet boats with Surf Rescue personnel on board, and four ground crews are searching the area between the Taramakau bridge and Kumara-Inchbonnie Rd. Image / Google Maps

"He was that weak he could not even hold on to it," Bagnall said.

"The current was running about 8 to 9 knots, so we were flying backwards with ours skids just on top of the water."

It was extremely difficult, the semi-flooded river was dirty due to recent rain and snow melt, he said.

It was easy to lose sight of the horizon while flying backwards in moving water and they could have become disorientated, he said.

But they only had one chance at getting him, his arm reached up and then he slipped under, Bagnall said.

"He just disappeared backwards into the murky water," he said.

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"You couldn't see more than six inches underwater."

They immediately knew they had to try to save the other man, he said.

Bagnall's crewman straddled the skid with his legs in the water and reached out to him.

"Once we had a grip on him we dragged him backwards and sideways through the water all at the same time," Bagnall said.

"It felt like an eternity."

They were not trained for a rescue like that and he never thought he would be in a situation like that, he said.

"It's just instinct really."

Bagnall said they flew further down the river to try to locate the missing man but it had become too dangerous to attempt any type of recovery.

Police were first called to the scene, about 1km upstream from the William Stewart Bridge, about 2.25pm yesterday.

The rescued man was treated by an ambulance at the scene and taken to hospital.

Bagnall has been a pilot for 15 years and credits instinct and close communication with his crewman for what they were able to do.

The search for the missing man resumed this morning with about 25 people scouring the area.

Four jet boats with Surf Rescue personnel onboard and four ground crews started searching the area between the Taramakau bridge and Kumara-Inchbonnie Rd about 9.30am.

The search was suspended overnight yesterday.