"I was only in Christchurch until I was about three or four," he admits. "That's when my dad, who was a carpenter, got sick of getting up early in the winter and decided to move us to Australia. I've been here ever since but I do frequently visit friends and family still over in New Zealand."
So in reality he's a Kiwi in name only? "We actually call ourselves Kaussies now," Tonkin laughs. "You know, half Kiwi, half Aussie."
But Tonkin passes the ultimate test of nationality with flying colours: "If it's New Zealand versus Australia on the sports field, I always follow the Kiwis, mate; always got a soft spot in my heart for them."
As a result, transtasman sporting fixtures offer the opportunity for some friendly sledging between Tonkin and Bullpitt, who met while working at the same school 15 years ago. Much of their relationship seems to be based on a respectful rivalry, and Tonkin says his interest in cooking was prompted by their matey one-upmanship.
"Paul is definitely the better cook," he says. "He's always done all the cooking in their house, his wife has never done any, and so whenever I went around it was always Paul preparing the meals. That's what got me taking up the utensils. After I got married, we started having Tuesday night family dinners, where three or four different families would take turns cooking and trying to out-do each other."
You'd think that shared competitive spirit is why the friends entered MKR, which Tonkin had watched on and off "without being a die-hard fan", but he's not sure.
"That's still a little bit of a mystery," he laughs. "We do know it involved the barbecue and a couple too many beers. Then next thing we were filling the online form out and sending in a short video of ourselves. So we really can't pinpoint whose idea it was, but I'm going to say it was Paul."
This season of MKR was shot in August and September last year and finished screening in Oz last Tuesday. Tonkin remains sworn to secrecy to protect the enjoyment of those Kiwi viewers able to resist googling the result. That means a lot of questions - such as how long Tonkin and Bullpitt were away from home and whether they and their families found that absence difficult - fall into spoiler territory.
It's okay to inquire what impact the show has had on Tonkin's cooking prowess, though.
"Oh, Mate, it's improved dramatically," he says. "Paul and I don't pretend we're chefs or anything like that. We just love food. At the start, we were intimidated by all the food knowledge people were displaying. I didn't know what half the words used meant and I said to Paul, 'Mate, I'm really worried I'm going to come across as looking stupid here.'
So it was clear right out of the gate that our learning curve was going to be steep.
"Making up for that is we're a pretty good team," says Tonkin. "We've worked together with troubled kids for seven years in the same room, so we know one another inside and out."
My Kitchen Rules screens Mondays, Tuesdays and Wednesdays at 7.30pm on TV2.