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Kobori led by three shots with six holes to play in the final round, thanks to a short game masterclass featuring a chip-in on the 12th.
But Masterton's Mountcastle rallied down the home stretch on Sunday, with an eagle on 14 setting up a tight finish. Both players were affected by nerves, bogeying 15 as the pressure intensified.
Mountcastle handled the heat marginally better, picking up the decisive shot on 16 to take the lead after Kobori’s tee shot found water, prompting a double-bogey.
Auckland 14-year-old Joshua Bai finished five strokes adrift of Kobori in outright third.
In the women’s competition, Auckland’s Aileen Du cruised to an eight-stroke victory after starting the final round with a six-shot buffer over Canterbury’s Maddie May.
Du carded an even-par 72 and the long-hitting May a 74, while Lizzie Neale (Nelson) was third on countback.
Waitikiri’s Brett Turner and Olivia Frost (Invercargill) claimed the masters titles from Jonathan Stieller (Weedons) and defending champion Kim Moggach (Waitikiri) by a stroke and 11 shots respectively.