After four days without food, a trouser-less tramper suffering severe hypothermia and possibly just hours from death was rescued by a helicopter crew - and later thanked them with a song.

Deep in the ranges of Te Urewera, about 11km west of Lake Waikaremoana, the man set off an emergency locator beacon before descending into the confusion of extreme coldness.

The Taupo-based Greenlea Rescue Helicopter was sent to look and spotted him through the mist at just before 4pm on Friday.

"He wouldn't have survived the night. It would have been surprising if he had survived most of the evening," pilot Nat Every told the Herald on Sunday. "He was wet, cold and semi-naked. The weather was cold and damp and deteriorating."

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The rescue service said the man was found in a clearing in the bush near Te Waiotukapiti Hut, above the flooded Waiau River, about 150m from the beacon's initial co-ordinates.

"What from above initially looked like a man with possibly a rolled ankle, transpired to be a trouser-less, boot-clad man with severe hypothermia, huddling into the foetal position - and a remarkable tale of survival," the rescue helicopter service said.

Otherwise uninjured, the man appeared to have been in the bush for several weeks, the rescuer service said, after he became trapped by rivers swollen with rain.

The rescue helicopter crew treated the man for hypothermia. Photo / Supplied
The rescue helicopter crew treated the man for hypothermia. Photo / Supplied

He had run out of food four days before his rescue and had been trying to make his way to safety when he was swept downstream trying to cross the river.

The man got back on to dry land and was making his way to the hut near where he was found.

This required a second river crossing and he was swept further downstream in the cold water.

Physically unable to get to the hut, the man activated his locator beacon.

After the rescue helicopter arrived the man was dressed in dry clothes and placed in a thermal electric sleeping bag before being flown out to Taupō hospital with a very low body temperature, the rescue service said.

"Despite his initial condition and obvious confusion, later in the return flight we were thanked multiple times and serenaded with a delightful song from a man that 30 minutes earlier had resigned himself to dying next to that river, 20m from a hut, but unable to reach it.

"Upon emerging from the murk and into the blue skies toward Taupō on our return flight, we were alerted to the wonderful views of Mt Cook and the Southern Alps."

Every said the man, aged in his late 50s or early 60s, had been hiking and appeared to be well equipped, apart from his lack of food.

He believed the man had taken off his trousers to cross the river, then slipped into hypothermia.

He was taken to Taupō Hospital. A spokeswoman said the man was treated and discharged from the emergency department.