Hard on the heels of a 10-day training camp in Invercargill, where he clocked up 130km, McMahon showed he has the form for a sub-60min 3.8km swim when he finished ahead of dozens of competitors in the Swim the Course race in Lake Wanaka on Wednesday morning to secure fifth.
He completed one lap of the 1.9km course in 28min, behind winner Aaron Barclay (16), of Gore, who took 25min.
Barclay, the 2008 New Zealand secondary-schools under-16 triathlon champion, is racing his third Challenge Wanaka and has been the youngest competitor for the past two events.
But this year McMahon is wearing that mantle after his mother, Julie, wrote to the organisers seeking special dispensation for her talented son, who is the top-ranked Otago swimmer in his age-group and 11th on the national scene.
McMahon has only raced in open water once before, at the recent Lake Hayes triathlon where he was fifth out of the water.
He specialises in freestyle events and, at the South Island championships last year, won gold medals in the 100m and 200m and a silver medal in the 400m.
"I just enjoy the racing aspect of it and going to all the big meets and stuff," he said.
The lake course got his full marks.
"It's better scenery than the black line at the bottom of the pool," he said.
He has swum the 3.8km distance many times in training but never raced it before.
McMahon learned to swim in the United States, where he was born, but took it up competitively with Scott Cameron's Dunedin Clubs Development Squad when his family returned to Dunedin to be closer to family seven years ago.
The former Balmacewen Intermediate pupil is a member of the Neptune Club and is now coached by Gennadiy Labara of the Otago Swim Coaching Academy.
He will be attending Otago Boys High School this year.
McMahon's team-mates are his mother, Julie (44), a Columba College teacher, who will cycle 180km, and his father, Sean (47), the managing director of Southern Colour Print, who will run the marathon.
Julie McMahon said the family was nervous but training had been going well, despite her husband's battle with injuries.
"We feel like our strongest leg is our swimmer. But for all of us, this distance is a first," she said.
She was impressed with her son's swim on Wednesday, given he was tired from his intensive training camp, which he finished on Tuesday.
"He wasn't very happy about getting up this morning at five o'clock to swim some more," she said.
