This is the second time the 46-year-old will compete.
This year he will be flanked by brothers Stevan and Tanu and three of their cousins, who Magasiva described as "big men" with a desire to "be healthy" as they approach the big five-oh.
"For most of us it's just about completing it and ticking a box because we're not built like triathletes. We are big men," he said.
"Most of us are over 45, so for us it's a long-term thing because we want to be healthy by the time we are 50 or 70. So it's kick-starting that health thing."
Magasiva has also made a pact to stop drinking, which he said is making training much easier this time round.
"I didn't enjoy drinking during Christmas and New Year's and I thought why not quit drinking.
"I think it just gets in the way."
He has also been watching what he eats and trying to get at least two sessions of swimming, running and cycling in a week.
"I started at 118kg. That's fat, that's humongous," Magasiva said.
The annual Dual event offers trail run or walk and mountain bike options alongside the off-road triathlon.
In its nine years the event has raised almost $300,000 for conservation and restoration work on Rangitoto and Motutapu by the Motutapu Restoration Trust.
"It's a great cause. [The environment] is something we need to look after, especially right now with the way the world is going.
"I want to make sure it continues for my kids and my grandkids."
