The fees for taking part in the Weetbix TRYathlon are too much of a hurdle for some families.
More than 300,000 six-to-15 year-olds have taken part over the years, but at $50 a child the entry fee is too rich for some budgets.
Principal of Nelson's Hampden Street Primary School, Don McLean, told Mike Hosking he doesn't understand why it's so expensive.
He says parents and educators are frustrated the event is heavily promoted, but not easily accessible.
Mr McLean told Mike Hosking it's soon obvious who can take part in the event - and who can't.
"The kids who have participated return to school in their TRYathlon t-shirts, and it becomes clear who can and who can't afford to go."