One year ago Lauren Irvine waited while her husband Brendan went under the knife to donate one of his kidneys to the couple's eldest son Harry.

The now eight-year-old Harry still has weekly check-ups and remains on heavy medication, but is healthier than ever before and to mark the "kidneyversary" he, Mrs Irvine and five-year-old brother Cooper will be cheering on Mr Irvine while he competes in his first Husky 'Long Course' Triathlon at Huskisson.

Harry was born with a large tumour weighing almost a kilogram on the base of his spine. Called a sacrococcygeal teratoma, it had grown through his bowel, abdomen and caused extensive damage to his tiny body including his kidneys.

"Because his kidneys were so bad and he got a big healthy kidney from me his recovery was a lot better … literally within hours he was doing well," Mr Irvine said.

"You always hope your kids are happy and healthy … but it's been a long time coming for him. He's put on about 5kg and grown about 20 centimetres."

The mammoth transplant operation wasn't Harry's first.

The couple have lost count of his hundreds of surgeries, beginning with a 14-hour operation when he was just four days old to begin the epic task of removing the non-cancerous tumour.

Mrs Irvine said Harry continues to face health challenges and needs a lot of assistance in year 2 at Canberra Grammar School, but he has defied all expectations.

"[They] said there was a good chance he may never walk or talk and he definitely did those things a lot later in life," she said.

"[But] to most people he looks like your average healthy child, he goes to school every day and he fits in with all his classmates which is amazing.

"He's the happiest kid you'll ever meet, he just thinks everyone is awesome and life's awesome."

Mr Irvine began training for the two-kilometre swim, 83 kilometre bike ride, and 20 kilometre run that makes up the epic triathlon just three months after the transplant operation – at a time when most donors are still resting up.

"In the first month your body does feel very foreign … you have to think about what you do. But now there's no difference," he said.

"They gave me all sorts of warnings, that it wouldn't be a normal life, but I was very headstrong in thinking I wouldn't let it affect me.

"I've always kept fit and healthy … but I haven't done anything sporting since school, [the triathlon] was one of those things that came out of the blue."

Unfortunately Harry could need another two kidney transplants later in life but Mr Irvine already has the perfect candidate in mind for the next one.

"Hopefully he'll be able to get 15 to 20 years and then Lauren will be able to donate one," he said.