Nine months later, Mr Yarnall, who turns 23 today, is in remission after two years in and out of hospital, and is about to step up his training programme for this year's edition alongside his new exercise partner Chemo.
A golden retriever pup, Chemo came into Mr Yarnall's life at the suggestion of a fellow chemotherapy patient who said her dog was a welcome companion during the long hours of treatment.
"Unlike Chemo, who was always running around trying to find people to pat him, her dog just sat there beside her," Mr Yarnall said.
"He's a bit too friendly, but great company ... I got him around Christmas time and he has been a good aid to recovery, getting me out and about and walking him."
While his colleagues were doing the Stadium Climb last year, Mr Yarnall was in an eighth-floor hospital ward receiving treatment to keep him alive.Eventually, he was well enough to travel to Christchurch to receive a bone marrow transplant, the donation coming from his brother.
The operation was a success and Mr Yarnall is home once more, studying mechanical engineering at Otago Polytechnic, and hoping to resume playing with his friends in the Andy Bay Falcons basketball team.
"The transplant has been amazing - I feel like I'm able to get back into basketball. I'm really enjoying my brother's blood," Mr Yarnall said.
"I am in remission, but I have to battle along with anti-rejection drugs. When you get a bone marrow transplant your body doesn't understand that it is to help and it semi-attacks.
"As his bone marrow attacks, yours fights back, but that's good because it also helps fight the cancer, and that's why it's done."
Before he fell ill, Mr Yarnall worked as an orderly at Dunedin Hospital. His colleagues rallied around him and entered two teams, "The Bloody Idiots' and "Haemodynamix", in last year's Stadium Climb - a sponsored walk up and down the stairs at Forsyth Barr Stadium.
"They managed to raise the largest amount of money out of anybody, and one of my nurses raised the largest amount of any person - she could probably be a team on her own," he said.
"I hope I can do my bit this year, do some training, and get up those stairs.
"I'm not sure if Chemo will be allowed in though."
Registrations are open for this year's Stadium Climb, which is on October 13.
