Austin will take part in the swim, assisted by his teacher aide Shannon Stowers, then cycle the rest of the triathlon on his new blue tricycle before walking over the finish line.
For years Austin has asked his parents to let him compete in the sporting event.
"He hears all the kids at school saying they are going to do the triathlon but he wasn't old enough or capable to do it before now," said Austin's mum Helen Manning.
But since his operation this year his physical strength had dramatically improved and he was extremely excited to take place in the event, she said.
"His T-shirt came, he wanted it on straight away and then he wanted to wear it to bed."
Mrs Manning said Austin had grown up watching his older brother on the side line of his rugby games.
"To be going to something Austin is doing for himself, that family members and friends can come and watch is just awesome."
Weet-Bix Kids TRYathlon co-ordinator Janine Bollee said Austin and the other pupils from St Thomas More started training at the start of Term 4.
Austin competing in the event was extra special, she said.
"His amazing teacher aide Shannon, along with his parents Helen and Brett, are making it happen for him on the day.
"It's really cool for him. He's in with the kids, the team and it's inclusive. The kids really get into it."
Mr Stowers said Austin had come ahead in leaps and bounds in his training and looked forward to the event this weekend.
Weet-Bix TRYathlon ambassador Valerie Adams said children got to have fun through play and exercise.
"Events like this help everyone involved make new friends and meet new people - no wonder more and more people are doing it every year."
