McEwan ended the four-round tournament as the only golfer in negative figures, being five under at the end, five shots clear of Vaughan McCall (Gore).
McEwan (St Clair) had won the event in 2002 and two years ago, and said it was satisfying to win again yesterday.
He was on 12-under after the opening two rounds on Saturday, thanks to two rounds of 65.
"It's always good to win on the home course and I was pretty happy to get through, although it was not great golf," McEwan said.
"I played really good yesterday [Saturday], and putted well, but today [Sunday] it was not great golf. I was off a little bit everywhere. I don't know what it was. I found it hard to concentrate at times."
He was eight shots ahead starting the second day but was inaccurate around the greens, and about halfway through the final round was tied with McCall.
But battling the tough wind, McEwan kept his poise and surged ahead at the end.
McCall hitting his tee shot out of bounds at the 15th in the final round handed the victory to McEwan.
The 29-year-old has been in good form this season, leading the Otago team to the semifinals of the interprovincial competition in December.
He put the improved form down to better practice and really concentrating when he did practise.
The bad luck story of the weekend went to Christchurch golfer Tom Turner.
He was right in contention before he hit a ball into the trees yesterday and it could not be found.
He parked his bag under the trees and went back to the tee to hit his third shot.
He hit that tee shot into the trees and the ball dribbled down out of the tree and hit his bag, incurring another shot penalty.
He finished the hole with a quadruple bogey.
