But as Vatuvei straightens up, peels off his helmet and flashes his legendary grin, he looks like the man who wept at his wedding and says it's not in his nature to hurt anyone intentionally.
He's been sparring with Nick Margison, a corporate-fight veteran who, at 193cm, is close to the height of Vatuvei's opponent (194cm). Margison runs a signage company but turns up each morning with hands strapped, ready to pummel and be pummelled.
Eyeing every move like a bald eagle is trainer Monty Betham, who is trying to make a boxer out of a rugby league player in just six weeks.
"Poker face, poker face ... perfect." Betham is schooling his prodigy to look neutral. No point in letting Murray know he's about to whack him.
"Whump!" Vatuvei catches Margison with a blow under the ribs. "Nice," Betham calls.
The buzzer sounds, the men touch gloves and retreat to their corners. Betham pours water into Vatuvei's mouth, the boxer's body pouring with sweat.
"That's perfect, now he's going to be in two minds when he comes back out. That's what I need from you."
Vatuvei nods, listening intently. Margison sits ringside, panting, pressing his ribs, wincing. "F***, I'll feel that one for a month."
In Hamilton, Eric Murray is going through the same torture each day at the Ringside Gym. Murray arrives in his sponsor-branded car and laces himself into new boxing shoes, a gift from a well-wisher who wants Murray to look the part on the night.
But the elite rower, who stood victorious with Hamish Bond on the podium at the London Olympics this year, knows it will take more than fancy shoes to pull this one off.
On the other hand, he's used to winning and he'll fight to the end. He's fit, big and strong - and agile. Boxing is as much about speed and stamina as it is about punching, experts say.
At this stage, the bookies favour the gold teeth over the gold medal but Murray and his trainer, boxing expert Rick Ellis, have some tactics up their gloves.
In the ring Murray is never still, dancing and prancing around on size-13 feet. He hopes stamina will carry him through as the big rugby league player tires.
Three two-minute rounds might not sound long, but the second-by-second concentration and physical exertion will leave both men mentally and physically drained by the time a winner is declared.
Murray's sparring partner is former rugby league player Mark Bourneville, a veteran Fight For Life boxer. Bourneville got Murray into this fight in the first place, so he obligingly takes the hits in the ring.
Jab, jab, punch. Left, left, right. Murray's long arms find their mark while Ellis talks non-stop from ringside.
"Step up ... throw your right hand, go, go ... use the round, push him off. Let's go, go again, good boy! Nice."
Bourneville has watched Murray improve enormously over nearly eight weeks of training, but it's the mongrel bit that has him worried.
While Vatuvei is used to the hard hits of rugby league, Bourneville says he's "not sure there are many punch-ups at rowing regattas". But, he says, it will be a "hell of a fight".
And Vatuvei isn't underestimating his opponent.
"I've heard he's really fit and he's tough. He will be going out there to win. I know he'll be punching me as hard as I'm trying to punch him."
Neither does Betham think Murray will be a walk-over. "Eric Murray is a gold medallist, unbeaten in four years at what he does. So we know what attitude and what mindset he'll bring to the ring."
With his league background, Betham also knows Vatuvei will struggle with a solo-focused sport where there are no lulls in play, no time to rest and no team back-up. "With boxing, you can't time in and out."
In Hamilton, Murray is having the same conversation with his assistant coach Eske Dost. Dost, 33, was an amateur boxer - 50 fights, ranked sixth in the world - for 10 years before she turned to training.
Murray is shadow boxing in the ring when Dost calls, "Don't let your guard down for a second, you can't slow down or you'll get hurt."
Dost will be in Murray's corner on fight night. Ellis has to step aside because International Boxing Federation rules say he can't coach amateur boxers and be involved in a "professional" boxing tournament at the same time.
Murray says he's not worried about fight night, even though his mates might be. To get to the top in any field, he says, you need to be ruthless.
"You have to go out and destroy the other person."
How they stack up
MANU VATUVEI
Age: 26
Weight: 107kg
Height: 189cm
Shoe size: 14
Nickname: The Beast. Nickname as a teenager, The Red Dragon because of the red car he drove.
Personal: Married to Jennifer, father to daughters Makayla, 6, and Savanah, 2. Lives in South Auckland.
Soft spot: Cries at sad movies and his own wedding.
Trainer: Monty Betham, former rugby league player and professional boxer.
ERIC MURRAY
Age: 30
Weight: 102kg
Height: 194cm
Shoe size: 13
Nickname: Eric
Personal: Married to Jackie, father to 16-month-old Zachary. Lives in Cambridge.
Soft spot: Gets teary during the national anthem on podiums and watching other winners.
Trainer: Rick Ellis, former boxer and trainer; and Eske Dost, former amateur boxer.
The Woodstock Honey Fight For Life will screen live on pay-per-view channel Sky Arena next Saturday (December 15) at 8pm, and live in Australia. Organisers hope to raise more than $150,000 for prostate cancer.
