The Year 5 race consisted of a 1km run, followed by a 3km bike ride and a 75m swim in the community pool the reverse order of typical triathlon events.
Johnston was coming 20th after the running section, but rode a brilliant bike leg to come through the pack to take the lead heading into the pool, where Caffell said many competitors were feeling the pace and seemed to be "swimming up and down on the spot".
It marked a good week for the Waverley children as two other members of the squad Camryn Caffell and Piper Neil came first and second respectively in Sunday's inaugural town triathlon in the 8-12 year grade.
"These guys gave up their morning tea break, while their mates were out playing games, for five weeks," said their principal.
"It was really good to go over there and see how we measure up, and we measure up really well."