Mr Mundy, wife Hayley and Harrison are well known in the Bay of Plenty. Mr Mundy teaches at Mount Maunganui College and has been involved in Surf Life Saving New Zealand in regional and national roles.
A fundraiser race is to take place at Mount Maunganui next month to help cover treatment costs.
Mr Mundy said he had been blown away by the support.
"It's brilliant and I think the best thing for us is actually the support. The money is definitely needed but knowing we've got this whopping big team of supporters back in Tauranga, it really does make a difference because we are struggling. His tumour has taken a step back to square one." The Mundys were preparing for "all outcomes" and looked forward to someone winning a trip to Fiji from the race, starting on December 4 at dawn.
House of Travel Papamoa, where Ms Mundy works, has offered an all-expenses cruise and flights for two to Fiji as a spot prize to someone pre-registered in the Sunrise Race for Harrison next month.
Friend and colleague Sarah Lockwood organised the event as a way of giving back to the Mundys, who would have been first in line to help out others in their situation, she said.
Sunrise race for Harrison:
Where: At the boardwalk at Leisure Island end of Mount Maunganui main beach.
When: December 4, 6am to 6.25am. Race begins at 6.30am.
Entry fee: $10 per person, for teams of three to four people.
Registration: at House of Travel Papamoa or on the day.