It was like MacGyver meets Rescue 1 when a tramper found a badly injured man lying in a stream in the Waitakere Ranges.

Simon Walker, 48, was part-way through an expected four-hour tramp at Huia on Monday when he heard something strange.

"I dropped down a hill and heard something. I didn't know what it was but my dogs had obviously picked it up. Once I got to the bottom that's when I managed to see something through the slats in the bridge that looked unusual - and then it moved."

He saw a man lying face up in the stream and tried to speak to him but the man was unresponsive. His shoes were on the bridge alongside his backpack, which had identification inside and a drink bottle. Mr Walker is a nurse and his medical training kicked in.

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"I went into the stream and tried to do a first aid assessment on the guy. It was obvious he was hypothermic - he was unresponsive and had a head injury."

An experienced tramper, he had a personal locator beacon because the area has no cellphone coverage. He set off the beacon and pulled the man as far out of the cold water as possible.

The Auckland Westpac Rescue Helicopter was contacted by the Rescue Co-ordination Centre and went to the scene but had trouble finding the exact location. Mr Walker used a signalling mirror that he was carrying to get the chopper crew's attention.

"I managed to get into some sunlight and using the heliograph mirror through a gap in the canopy I flashed the chopper. They spotted the flashing and that's how they found me."

Intensive care flight paramedic Rob Gemmell was winched down, examined the man and found he was hypotensive with a very low temperature and almost completely unconscious.

The man was yesterday in a stable condition at Auckland City Hospital.

Mr Gemmell is adamant Mr Walker's actions saved the man's life.