The keen hockey player was training for the Omanu Sticks a month ago when he fell, complaining of pain to his little finger and arm afterwards.
"He had what we thought was a hockey injury," Mr Mundy said. "That started to get worse. When he got to the stage where he had really bad pains in his hands we went to A&E.; We just thought we would go in, be told 'this is what you have done', but no."
Harrison was admitted and booked for an MRI scan the following day.
"They told us the MRI was 40 to 50 minutes long. They stopped after 5 minutes, saying he had moved around too much but in reality they had picked it up," Mr Mundy said.
"It" was a tumour the size of a softball on Harrison's C-7 vertebrae. It was part of the cancer located in his spine.
Mr Mundy said the diagnosis was "like having your heart ripped out". Harrison was put in an ambulance and taken to Auckland's Starship Children's Hospital on June 20.
He has since had 76 procedures to help fight the cancer, each one now represented on his string of Beads of Courage. Yellow and blue stars represent surgery. Rainbow cubes represent specialist appointments and a single dark red and grey bead - much bigger than the rest - represents bravery.
"I got to choose that one. I liked the colours on it," he said.
Harrison said he was glad to be home and had missed being with his school friends.
The family returned home on Friday and are now adjusting to a new life. Harrison's mum Hayley Mundy does not know when she will return to work at Papamoa House of Travel and Mr Mundy is taking advantage of school holidays from his job at Mount Maunganui College.
Harrison will need chemotherapy every 14 days for a year. Preliminary tests indicate the treatment has already reduced the size of the tumour, which presented its own risk of surgical removal because it was on his spine.
Mr Mundy said they were lucky to have picked up the cancer the way they did, otherwise it could have been "too late".
The Mundys have been inundated with support, including a $500 donation from surf lifesaving legend and octogenarian Sid Salek, who raised $5 for each kilometre he swam in Vanuatu this month.
"It's always been a big family [Surf Life Saving New Zealand] but when you get something like this, it's really humbling," Mr Mundy said.
Mount Maunganui College was organising a head shave event and a "Mundy Fundy" had been set up through Papamoa House of Travel to help the family through Harrison's treatment.
Details: www.surflifesaving.org.nz/news
What is Ewing sarcoma?
Ewing sarcoma is a form of primary bone cancer. Primary bone cancer is different from secondary bone cancer in that the cancer begins in the bone. In secondary bone cancer, the cancer begins somewhere else and spreads to the bone. Bone cancer is very rare. Primary bone cancer is even rarer.
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