The young overseas tramper who swam across a Southern Alps river in a raging, "wall-to-wall" flood walked calmly into a cafe and organised the rescue of his mates.

He and his four tramping companions had one channel of the Makarora River to an island and he swam the second channel to safety last evening. The four were rescued by a helicopter at 10.40pm.

A crew member on the Lakes District Rescue Helicopter, Ed Halson, said the four rescued trampers were uninjured.

"They were just cold, wet and very grateful."

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When asked if the usually crystal clear river was brown with the flood, he said, "It was battleship [grey] and huge - wall to wall."

The man who swam the swollen torrent appeared to be a German in his early 20s, said Casey Gordon, cafe manager of the Makarora Tourist Centre - which has a cafe, bar and accommodation - where the man came to seek help about 7.20pm.

The group of five had tramped the Gillespie Pass circuit and had yesterday come down the Wilkin River from Kerin Forks Hut to return to Makarora township.

"He was soaking wet and freezing. He was pretty scared and shaken up and lucky he got across; very, very lucky really.

"He kind of came up to the counter and was calm, for the situation. He said he had swum across; his friends were still trying to cross.

"He said he had been swimming all his life so he was more confident about it than them.

"Someone at the bar called the police."

Gordon said a woman who was at the tourist centre was the girlfriend of one of the trampers rescued from the island in the river. She attended to the swimmer, getting him some dry clothes.

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An Otago Regional Council rain gauge at the Young River North Branch, which feeds the Makarora River, recorded more than 100mm of rain yesterday, with the heaviest falls being in the late afternoon and evening.

Fire chief Careen Kemp, of the Makarora Volunteer Rural Fire Brigade, said locals knew not to try to ford the river when it had been raining heavily.

She said a commercial-jetboat driver saw the five young men, who were of various nationalities, earlier in the day and told them she could pick them up at about 2.30pm.

The men were not carrying tents.

After they were rescued they were taken to the fire station and given food and warm drinks.

The Lakes District Air Rescue Trust said in a statement that the rescue of the four trampers was delayed by poor visibility in the Queenstown/Wanaka area initially preventing the take-off of the helicopter equipped with night-vision goggles.

The rescue site on the island was lit up by the headlight beams from a fire truck on the township side of the river. A search and rescue "swift water" team had been on standby to rescue the trampers on an inflatable raft if the helicopter had been unable to retrieve them.

The trust said: "Rivers can become treacherous in this area and finding shelter to wait out the event is better than risking life."

In April 2015, a young American woman drowned on the Gillespie Pass circuit in Mt Aspiring National Park. Allison Willen, a 20-year-old student, was washed away near the foot of the pass when the Young River South Branch flooded the track.

Police search and rescue co-ordinator Sergeant Aaron Nicholson said at the time: "'She didn't cross the river. She hasn't made a bad decision to cross the river. She's just been walking the track and gotten to bits where the river has swelled over the track quite significantly."