The 65-year-old had been encountered by two hikers as he was heading to Welcome Flat Hut on Friday evening.
The hut is about 17km from the car park near the State Highway bridge across the Karangarua River.
The two hikers, also members of Search and Rescue, advised the man the track was closed because of the orange alert for heavy rain, but he became agitated and walked off.
Concerned by the man's behaviour, and inadequate preparation, they notified police, and the description matched a person, wanted by mental health authorities, who had also been in Haast in previous days.
SAR teams were dispatched to catch up with the man about 90 minutes after his interaction with the hikers.
A note dropped by the man was found as were footprints, but the SAR team arrived at Architect Hut, halfway to Welcome Flat Hut, at 1.30am with no sign of him.
Two more SAR teams were flown into the Copland Valley the following morning to cover the remaining area to Welcome Flat Hut but deteriorating conditions saw them pulled out in the early afternoon as wind speeds increased to 50 knots.
A dog team from Hokitika found some trampers who had earlier sighted a man on the opposite side of the Copland River to the track, near the bridge at Katau Creek.
"The man had somehow got into the water, been swept downstream and got himself out on the other side," senior sergeant Mark Kirkwood said.
A helicopter was used by teams to reach the other side of the swollen river which was "in a raging torrent".
With just two hours of light remaining, and amid heavy rain showers, the team located the man who was "very wet and cold".
He was provided with warm clothing, and flown to Te Nikau Hospital for assessment.
Mr Kirkwood said if the man had spent another night out in the elements it was likely he would have died due to exposure.
"The Land SAR teams, members of the public, Department of Conservation and police saved a life on this search."
- Meg Fulford
