In an outstanding 13-month spell, Ko has now won the New South Wales Open to become the youngest player, male or female, to win a professional event, the Canadian Open to become the youngest-ever winner on the LPGA Tour, and now a professional tournament at home.
"It was a pretty good day out there and I'm pretty excited to have my name on this trophy," she said. "I didn't cry at the Canadian Open so I've got no idea why I cried here, I guess it meant more.
"It's our national open so to win it means a lot. I'm not the sort of person who shows expression or feelings that much but I guess I did here."
The 15-year-old Korean-born New Zealander approached the last hole in a tie for the lead with American Amelia Lewis at 10-under. But Lewis, in the penultimate group, three-putted from 25 feet for bogey and Ko, playing in the last group, made par to complete a final-round 68.
Ko finished at 206 for the three-round tournament at Clearwater Golf Club. Lewis finished with a 6-under 66 for a 207 for the tournament, which is co-sanctioned by the European and Australasian women's tours.
Cheyenne Woods, the niece of Tiger Woods, finished in a tie for 25th at 3 under while former world No. 1 Laura Davies was 51st.