Mr Matthews, one of the Motatapu event directors, said when he bumped into Mr Hawke he asked if he would be returning to compete in any of the Motatapu events this year.
Mr Hawke responded by saying not until he could do the "triathlon".
"I was a little lost and asked him 'what triathlon'?"
"Gavin said: 'Swim in Glendhu Bay, bike the Motatapu mountain bike [course] and run the Miner's Trail'."
Mr Matthews said he was "a little stunned".
"It was my event, but it had never occurred to me to run a triathlon over it.
"He beat me fair and square to the idea."
Mr Hawke has competed with distinction in three of the four Motatapu events - the 42.2km off-road marathon, the 47km mountain bike and the adventure run.
He said having done all three events "[I] was thinking, if I'm going to do it again, I either have to do it naked or combine all three".
Mr Matthews said he immediately agreed the event should be run as invitation only and limited to 50 competitors in its first year.
The race will begin with a 5km swim in Glendhu Bay, Lake Wanaka, before the 47km mountain bike ride from Glendhu Bay to Arrowtown through three high country stations, and end with a 15km mountain run over the historic Miner's Trail.
It would be a "true off-road long-course triathlon" and was expected to take the top competitors more than four hours to complete.
"This means it will be slightly longer in duration than a half-iron man distance triathlon," Mr Matthews said.
Mr Hawke described it as the most outstanding off-road triathlon in New Zealand.
"A beautiful, cool, flat swim, a fast non-technical mountain bike and then a run at the end which will become a true test of someone's running ability."
Mr Matthews said Mr Hawke would thrive in the run.
"Gavin is a fierce competitor and was one of the best runners around in the '90s."
