McCall (19) carded five birdies and an eagle as rain and hailstones made scoring difficult for Otago and Southland's top golfers.
He began the final round one shot behind Brent McEwan (St Clair), who faded, giving McCall an eight-shot winning margin. McEwan finished in a tie for second on 294 with Cody Harper (Invercargill) and a rapidly improving Luke Murdoch (Otago).
The win was a fillip for McCall who is one of New Zealand's top junior golfers and heads to Singapore with a national team next week.
Play was suspended halfway through the final round when lightning and a blanket of hailstones on the greens made putting impossible.
McEwan looked set to take the honours when the event looked like being cut to three rounds. But play resumed after half an hour and McEwan drifted further back on the remaining twelve holes.
McCall, McEwan and Harper shared the first round lead on 71 with Tyrone Ratahi (Otago) a further shot back on par figures.
Murdoch and Brandon Hodgson (Otago) were handy on 73.
Harper made the pace in the afternoon round with a par 72, giving him the overnight lead over McEwan by one. McCall and Murdoch were two further shots back, while Ratahi was four off the pace.
Harper scored 76 in the third round, giving the lead to McEwan, with McCall also one shot back.
McCall took the lead on the first hole of the final round with an eagle 3 and took control with birdies on the third, sixth and eighth holes.
He made two further birdies on a faultless homeward nine with threes on the 10th and 13th holes.
Kayne Wardell (Otago) tied for 11th overall to win the division two gross on 307 by nine shots from Samson Kim (Queenstown).
Joshua Kahukura (Omakau) won the third division gross, also by a healthy margin.
