The South Sydney back-rower, originally named in his natural position out wide - made the late switch into dummy-half after Siliva Havili was ruled out of the contest - starring in the role.
Havili picked up a calf injury in the Rabbitohs' semi-final victory over the Cronulla Sharks, missing their preliminary final loss to the Penrith Panthers as well as the early stages of the tournament.
Koloamatangi made the late switch an hour before kick-off with limited hookers in the side, young Warriors' hooker Taniela Otukolo the only other recognised No. 9 in the Tongan squad.
Despite playing an unfamiliar position, Koloamatangi still managed nearly 150 metres, 34 tackles as well as scoring the match-winning try, his first for Tonga.
With Havili still considered a 50/50 chance of recovering from his calf injury to face Wales this weekend, head coach Kristian Woolf refused to rule out continuing the experiment throughout the tournament.
"We'll figure out whether we persist with it through the week," Woolf told AAP.
"We just wanted to put someone in there who had the skill.
"He did a great job I thought, some of his passes weren't right on point but he can defend there, keep it simple and he got us off to the right start."
Just look at what it means!
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While a cloud surrounds Havili still, the picture is slightly clearer for the return of Will Hopoate, who is aiming to make his comeback in the final pool game.
"He (Hopoate) won't play next week but he's a real chance against the Cook Islands," Woolf added.
"I know what sort of class Will's got and with St Helens we never lost a game with him this year.
"That speaks volumes of his class and his influence on this team and I know how much it means to him to play for Tonga."