The 26-year-old Belgian was attempting to walk across the river delta between Glenorchy and Kinloch when he was caught in the strong current.
Kinloch Lodge co-owner John Glover said he launched his boat after his son heard someone calling for help.
He pulled the man from the water about 200m from shore.
"He was drifting down the lake, hanging on to his backpack, and looking very sorry for himself.‘‘He was very relieved."
The man, who was planning a few days’ tramping in the area, had crossed most of the delta but got in trouble in the river’s main outlet.
If the man had drifted past Kinloch without being noticed, the current and wind would have pushed him out into a remote part of the lake.
"Another 15-20 minutes and he would’ve been in serious trouble," Mr Glover said.
He had rescued several people in a similar predicament over the years, including people tipped out of kayaks, fishermen and windsurfers with broken gear.
Queenstown Lakes harbourmaster Marty Black said it was yet another case of a Good Samaritan averting a tragedy, and it was "sheer luck" the man’s cries for help were heard.
Attempting to walk across the delta was ‘‘stupid’’. Its hazards included sinking sand, very cold water and strong currents.
