The Canberra event was the fourth leg of the 17^th annual triathlon and has become so popular organisers had to expand the course at the Australian Institute of Sport to accommodate the 1525 children who wanted to swim, ride and run their hardest.

"We are at absolute capacity at 1525 this year," the triathlon's spokeswoman Sheree Dell said.

"We are in the process of seeing how we can extend it further, so we have an opportunity to offer this event to more kids in the Canberra region."

Seven to nine-year-olds swam 75 metres, rode three kilometres and ran 500 metres in the triathlon, while the 11 to 15-year-olds took on a doubly-tough challenge of a 150-metre swim, six-kilometre cycle and one-kilometre run.

The nine to 10-year-old category was the most popular, Ms Dell said.

"I think the thing that is appealing is that everyone is rewarded equally. They all receive a medal when they cross the finish line from a TRY Hero … for just giving it a go," she said.

"It's really about boosting their self-esteem. Not only about encouraging activity, it's also about them getting active and having fun with their friends.

"This is a good way to do it, to try three different disciplines of sport."

Canberra's TRY Heroes, champion BMX rider Caroline Buchanan and Brumbies and Wallabies players Henry Speight, Stephen Moore and Matt Toomua, handed gold medals to the children as they crossed the finish line.

"We've been overwhelmed with the amount of support we've got," Ms Dell said.

"It's about encouraging kids to get active at a grassroots level."

With help from the New Zealand events, organisers aim to break the world record again for the largest under-16 triathlon series of 40,000 participants when all 14 Australia events wrap up later this year.

"We've definitely grown year on year, so we know that we're looking in good stead to break the record again this year," Ms Dell said.