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Mackay went to the 21st hole to win the semifinal over Liam Belneaves after both players holed great putts to be all square after 18 holes.
MacKay then birdied the third hole at the Balmacewen course to go into the final against defending champion Jack Turner. Turner beat Joseph Hancock five and four.
Mackay (21) made a great start to the final, birdieing three of the first four holes and went on to win six and five. He had to contend with hail and torrential rain on the seventh hole and the players, and caddies, sheltered in the trees for about 20 minutes as the heavens opened.
Mackay came out and continued to hit the fairways, putted well and was too good for Turner.
Mackay started playing in Oamaru and had been playing for Aorangi until recently when he decided to play for Otago. He worked at the Timaru Golf Club for Cromwell professional Willie Moore, and has linked with the Cromwell club.
"I've always liked playing here. I know all of the guys and one of my good friends [Inia Logan], this is probably his last time playing for Otago,'' he said.
It was a good win yesterday as he had only entered one championship before, last year when he won the division two section.
"It's a good feeling. I didn't really give him anything.''
Mackay said he usually struggled in strokeplay around the course and with rounds of 71 and 78 on Saturday, qualified just 10th for the matchplay.
But he got on a roll and was too good.
In the women's section, Queenstown youngster Sumin Kang beat Susan Greig, of St Clair, two up.