"When she first started she was quite a big lady, and she's come a long way. She's helped me get ready physically and mentally for the TRYathlon."
Ms Dunkley said the pair had been brought even closer together since starting to train for events, which was a natural progression from her decision to get healthier five years ago.
She has since lost 32kg.
"I was doing it for Zach in the first place, I wanted to change and be healthy for him, and I'm stoked that it has inspired him to be more active too."
Zach said it would be his first TRYathlon, but he cycled to school every day and was confident he would be "pretty fast" on the cycling leg of the race.
"I feel prepared for it. I'm looking forward to getting a free T-shirt and think it will be a lot of fun."
Ms Dunkley said she was shocked when Zach told her she was his inspiration for entering, but believed the experience would be good for both of them.
"As long as you're having a go, that's what matters, and that's what I've been telling Zach.
"He wasn't really into fitness before and I don't think he felt like he was good enough to do an event like this, but we have been having so much fun training together." The Sanitarium Weet-Bix Kids TRYathlon is taking place in Rotorua on December 13.
The events are open to children aged between 7 and 15 and consist of swimming, cycling and a run.
-To register go to www.try.weetbix.co.nz.