He said his putter was running hot during the final round, which helped him finish at four-under par for the tournament.
"I putted really well today, probably the best I've putted in a long time."
Hillier, from Te Puke, is currently on a golf scholarship at the University of Kansas and will return to the US later this month.
He said he was rapt to be added to the list of names who have won the tournament in the past, including Rotorua professional Danny Lee.
Koong was disappointed after the match and said he struck his first putt too hard on the 18th, when he had two shots to win the tournament.
"I was just trying to safe putt it, but rammed it," he said. Impressively, Koong tied Danny Lee's course record during the tournament, shooting 62 (eight-under par) during his second round on Saturday.
"[On Saturday] I was able to get my second shot on the green then putt birdies, but today I just couldn't do what I wanted to do."
Meanwhile, things were a lot more one-sided in the race for the women's title, as Auckland's Yena Chang finished seven shots ahead of her nearest rival Rotana Howard (Rotorua).
Chang, 18, said she was very pleased with the victory despite having trouble with her driver.
"I think my putting really helped as well as my game plan this week, which was just to have fun," she said.
Chang, who was born in South Korea but grew up in the United States, finished at three-over par.
Results:
Men: 1 Charlie Hillier 206* (70, 67, 69), 2 Kevin Koong 206 (71, 62, 73), 3 Nathan Kim 209 (72, 70, 67).
Women: 1 Yena Chang 219 (76, 69, 74), 2 Rotana Howard 226 (78, 73, 75), 3 Carmen Lim 226 (74, 74, 78).
*Won on a playoff.