Almost a decade after representing the International team at the inaugural Junior Presidents Cup in 2017, the Canberra native endured a roller-coast final day at Royal Port Moresby Golf Club, where NRL star and avid golfer Jarome Luai was a special guest.
A shot behind Queenslander Louis Dobbelaar at the start of Sunday's final round, Armstrong surged to the front with an eagle at the par-5 ninth, followed by a birdie four at the 10th.
Victory certainly didn't come easily for the 27-year-old, who had previously accrued six top-five finishes, the best of which was a tie for third at the Heritage Classic in April.
A bogey at the short 13th on Sunday was followed by a double-bogey at the par-4 14th to bring Dobbelaar (72) and Victorian Lachlan Aylen (71) back into the frame.
Armstrong steadied with pars at 15 and 16, then delivered an incredible stinger shot out of heavy vegetation on 17, draining the birdie putt from 10 feet to be one clear with one to play.
"I don't know if anyone saw, but I actually gave myself a fist pump after hitting that," Armstrong said.
"I said to myself a few expletives when I was walking off that tee there, just trying to fire myself up to get the job done. Fortunately, it paid off."
Aylen finished with a bogey and Dobbelaar could only make par, meaning a par was enough for Armstrong to close out a round of three-under 69.
Armstrong finished at 14-under, two ahead of Dobbelaar and Aylen, who shared second.
"I don't even know what to say. It probably won't be until I get back to the hotel or probably even later that it'll sink in properly," said Armstrong of finally breaking through.
Admitting to periods of self-doubt as he tried to establish his professional career, Armstrong can now look forward to the Australian PGA Championship and the Australian Open as he assumes the No.1 spot on the Order of Merit.
Without status for the 2026/27 season, a tie for second will help Aylen secure more starts in the months to come, while Dobbelaar looks poised for a rewarding summer.
In the women's event, New Zealand rookie Rebekah Blackwell-Chin came from behind to claim the second PNG Women's Open.
A graduate of the WPGA Tour of Australasia Qualifying School in December and a two-time NT Amateur champion, Blackwell-Chin admitted she was caught off guard by her eight-over 80 on the first round of the two-day event.
She turned that around in emphatic fashion on Sunday, making six birdies in her opening 10 holes in a round of one-under 71 to finish five strokes clear of Kathryn Norris, with defending champion Amelia Mehmet-Grohn sharing third with Kay Bannan.
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