Shin mixed three birdies with two bogeys in a final-round showdown with Ko and Tseng of Taiwan, and added the Australian title to her two British Opens among 37 professional victories.
Ko's share of the lead evaporated quickly when she began the final day with a double bogey and bogey. However, the 15-year-old who was born in South Korea and is based in New Zealand fought back to reclaim the joint lead after 13 holes before Shin dismissed the challenge.
Shin effectively clinched her victory on the 14th hole when, tied with Ko at 16-under, she holed an incredible pitch from deep rough behind an advertising billboard to make birdie.
"I'm really surprised," Shin said. "There are so many good players here, so I'm really happy to play well. I can't explain at the moment. I just feel excited."
Ko finished 3-over for the day and 14-under for the tournament, in outright third place. She was trying for back-to-back wins in professional tournaments after winning last week's New Zealand Women's Open.
Ko has won three professional tournaments in the last 13 months, taking the New South Wales Open to become the youngest winner of a professional event, then winning the Canadian Open, to become the youngest winner on the LPGA Tour. She struggled to overcome her poor start Sunday and was never able to comfortably settle into her final round.
"I had a few struggles with my drives but, I can't play good every single round," Ko said. "Third in a professional event is a pretty good result and I can't say I'm not fully pleased."
Meanwhile, Tseng shot the best round of the last day - a 7-under 66 - to dash past Ko into second place, two shots behind Shin.
"I was always feeling good here and I tried my best today," Tseng said. "I got some pretty good lucky bounces, too. I didn't drive as well as the last few days, but I got some good bounces, some good kicks on the fairway, hit a tree, bounced on the fairway so it's kind of really lucky."
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