Women's world No 1 Xuetong Cai, of China, led from the start with an impressive score of 92.8 to win the women's competition, which was the last event on the 2011 Winter Games programme.
A total of 12 men and six women battled it out in the finals.
Korpi, competing in his first Winter Games, stole the limelight with a performance which included a backside rodeo alley-oop 540 nose grab, followed by a backside 900 mute grab into a frontside 1080 tail grab cab 1080 nose grab, finishing with a stalefish.
He said changing the order of his tricks, in particular pulling off the backside rodeo at the beginning of his run instead of at the end - which was "more risky", played a key part in his success.
"The competition went really well for me today. I decided which run I was going to do yesterday, stuck to it and it paid off. I didn't do anything new today but changed the order of tricks and it paid off," Korpi said.
Korpi has based himself in Wanaka for training during southern hemisphere winters for the past five years.
"Wanaka is so much fun. You can go ride in the morning and play tennis or golf in the afternoon."
Yiwei Zhang, of China, was second place, with a score of 90.8, and Dimi de Jong, of the Netherlands, placed third with 88.0 points.
Johnstone, who finished in fourth, had no complaints with his performance, despite being bumped off the podium.
"It's always good to come out of the southern hemisphere with a result but I'm still happy with how I rode, so it's a good day," he said.
There were no surprises in the women's field, with Cai's winning run made up of a frontside indie, back 540 mute, frontside 7, cab 3 mute grab and frontside 540 melon grab. World No 2 Holly Crawford, of Australia, took second place with 88.5 points and Ursina Haller, of Switzerland, was third on 77.3 points.
Crawford said after hitting her head "pretty hard" during training yesterday, she was thrilled with her podium placing, which was a "great start to the season".
"It's a bit of a warm up [to the northern hemisphere winter] so I was sort of not throwing my bigger tricks.
"It's early in the season and there is no point going out too hard and injuring yourself."
She said the Chinese women in the competition always put up a strong fight and raised the level of competition.
"They're just getting better and better."
Judge Steen Webster was impressed with the level of competition.
"Today was excellent with the riders really stepping things up a gear going into the final. Janne Korpi's winning run was just outstanding and a pleasure to watch," Webster said.
"There were no new tricks today but we did see a lot of doubles, which is great. Although it's fast becoming a stock trick, it's still not necessarily a competition winner and if riders chose to perform this trick it needs to be executed with real precision."
