Ussher won in a time of 4hr 6min 26sec.
The former Queenstown athlete, now living in Nelson, was using the race to test his swimming, which he regards as his weakest discipline, now he has added long-distance triathlons to his programme.
The six-leg off-road race was the first in a national series and featured Olympic athletes, world champions and national surf ironman champions among the 500 entrants.
The event started with a 1.9km ocean swim, followed by a 13km ocean and river kayak, a 12.5km mountain run, 18km mountain bike, 1km board paddle and a 2km beach run.
Ussher was the 29th swimmer to exit the water, in 32min 41sec, behind 18 male team swimmers, five individual male competitors and four female swimmers.
Ussher was about five minutes off the top pace set by Nick Cuthbert of Australia (27min 50sec), who was swimming for the Epic Kayak team, and two minutes behind the first individual man, Ben Allen (30min 21sec).
Ussher had been tagged as the race favourite and quickly moved into second place during the kayak section before taking the lead from 2007 defending champion Guy Andrews during the run.
He was able to fend off fast finishers Jarad Kohlar (second) and Sean O'Neill (third) to lope home with a four-minute advantage.
Australian teenager Nicole Rutland won the women's race in 4hr 53min 14sec, ahead of Karla Gilbert (5hr 8min 55sec) and Deanna Blegg (5hr 17min 51sec).
