With rounds of 70, 71, 71 and 70, McEwan finished three shots ahead of Millbrook professional Ben Gallie. James Anstiss (Queenstown) and Matt Tautari (Southland) tied for third.
Invercargill professional Cody Harper came in fifth, one stroke ahead of defending champion Mark Brooks (Queenstown).
McEwan has gained his top form after only two weeks of competitive golf following his wedding and honeymoon last month.
Gallie lost his chance to win the title after he turned six over par in the final round. But he romped back into contention with an incredible homeward nine, scoring an impressive 29 with seven birdies and two pars.
The women's title was a tight tussle all the way, between Abigael Crawford (St Clair) and Tracey Storer (Belleknowes).
Crawford took the honours by a single stroke when Storer faded on the final four holes.
Local golf guru Simon Boland won the masters (over-40) title with rounds of 75 and 76, beating Southland's Karl Mariu by four shots.
Gallie set the pace in the opening round with a five-under-par 67. The feature of his round was back-to-back two's on the par 4 ninth and the par 3 10th holes.
Daniel Elder (Otago) was the leading amateur at that stage, at three under par, carding seven birdies in his round of 69.
McEwan and Matt Tautari (Southland) were in hot pursuit one shot back, while Duncan Croudis posted the only other sub-par round with 71.
Harper posted the lowest round of the afternoon on Saturday with a four-under-par 68. He had finished with 73 in the morning and was one over again after 12 holes, before he holed out from 158m with a 7 iron for an ace.
This was to be the catalyst for a sub-par finish and he birdied three of the last four holes.
But this still left him in second place, as Croudis took the halfway lead at four under on 140 with an afternoon round of 69.
Brooks was four shots off the pace after two par rounds while Queenstown junior Sungwoo Han moved up the leader board with a one under 71 on Saturday afternoon.
This placed Han in a three-way tie for sixth with Anstiss and first round leader Elder.
On Sunday morning Croudis had five consecutive bogeys on the front nine and relinquished the lead after finishing with a disastrous 78.
McEwan was quick to take advantage of this lapse, shooting his third sub-par round to take a two shot lead over Gallie going into the final round.
Gallie scored a 70 in his third round, edging out Harper by a single shot, while Brooks was five shots off the pace on 217.
The bogey-ridden round cost Croudis dearly and he slipped back to 218, to be tied for fifth with Anstiss.
