Cairns was happy with his race, the fastest on the new course.
"I felt a bit tired, though, from yesterday," he said.
Cairns is gearing up for the national secondary schools' triathlon championship.
Thomas Christison was the second senior boy home yesterday with Hadlee Dyer third.
Sophie Hogan was named the senior girls' champion, completing the sprint in 1h 9m 5s, Dani Baker came in second and Bella Langlands-Grieve third.
Sarah-Kate Birkett won the junior girls' race (200m swim 14km cycle, 2km) in 41m 42s with Alia Wentz second and Hana Peacock third.
Will Tidswell was the first junior boy across the line in 34m 35s with Charlie Tattersfield and Kairon Pimms next, respectively.
Graeme Buscke (open) took the men's club championship title over the sprint distance in a time of 1h 3s, after enduring tough trying conditions at thee Kellogg's Nutri-Grain Ironman New Zealand last week in Taupo where he was third in the 35-39 age group but that did not slow him down.
Runner-up Jeremy Natusch (open) was two minutes behind while Josh Page (open) came third.
Eva Goodison (junior) made a triumphant return from injury, taking the women's race in 1:6:34.
Naomi Fergusson (open) was runner up in her second race of the weekend, also competing in Wellington the day before.
Linda Exeter-Grant (vet 2), who won the 50-54 age-group category of the Taupo Ironman, was third.
