World No 954 Ryggs Johnston held off the local competition to claim a stunning ISPS Handa Australian Open win by three strokes at a wet and wild Kingston Heath on Sunday.
South Korea's Jiyai Shin won the women's trophy in the dual-gender event by two, overcoming defending champion Ashleigh Buhai to claim the title 11 years after her first.
While Shin made sure of her win by storming to a seven-stroke lead midway through, American rookie Johnston wielded magic on the back nine greens to see off an unlikely challenge from unheralded Western Australian Curtis Luck.
Jasper Stubbs and Marc Leishman were a shot further back.
Johnston, the 24-year-old from Libby, Montana, carded a final round 68 for an 18-under total of 269, winning in his second start on the DP World Tour and claiming exemption to next year's Open Championship.
"I turned up pretty tired already from the all travel, Q- school and everything," said former Arizona State golfer Ryggs.
"I couldn't get a practice round [at Kingston Heath] with the weather. I didn't really have any expectations and it probably helped in the end.
"It hasn't quite sunk in yet but it's an amazing feeling."
Named after Martin Riggs, the fictional lead played by Mel Gibson in the Hollywood cop movie 'Lethal Weapon', Johnston started the day with a two-shot cushion with Lucas Herbert but both were reeled in by Luck.
Luck shot into the lead with his seventh birdie at the 14th but Johnston, playing three groups behind, curled in a long birdie at 14 to draw level.
Johnston then drained another from beyond 30 feet in the next hole to snatch back the lead before Luck fell away with back-to-back bogeys on 17 and 18.
Though Luck's challenge fizzled, the 28-year-old booked a ticket to The Open, while LIV golfer Leishman took the third and final exemption on offer.
After leading, or having a share of the lead after the first three rounds, Herbert had a testing day on the greens as he shot a 74 to slide to 12-under and a tie for fifth.
Defending men's champion Joaquin Niemann could not repeat his 64 from Saturday which saw him storm back into contention and could only manage an even par final round of 72 for a 12-under 275 total and a tie for sixth.
The 2022 British Open champion Cameron Smith shot 71 Sunday to finish at three-under 285 and in a tie for 39th to end his four-tournament stint back home winless.
Earlier, Shin produced the shot of the day as she chipped in from 102 yards for a sensational eagle on the fourth hole, having whacked her tee shot into the scrub.
The eagle was sandwiched by birdies at the third and the fifth which opened a six-stroke lead over home hope Hannah Green, her closest overnight challenger, who promptly crashed with a triple-bogey on the seventh.
Shin notched another eagle on the 10th to stretch her lead to seven shots over Buhai.
The South African charged hard with five birdies on the back nine as Shin wobbled with bogeys on the 15th and 17th, shrinking the lead to two strokes at the 18th.
Looking to force a play-off, Buhai slid a long birdie putt by the cup, allowing Shin two putts for victory from six feet.
Shin drained it on the first attempt to finish with a final round 70 for a 17-under total of 274, claiming her 65th professional win across various circuits.
"It wasn't easy but now I can breathe," Shin said greenside before being doused in water by fellow South Korean golfers.
"I had a pretty solid full week and I never had a bad day in this tournament so that's why I love to play golf."
South Korean amateur Hyojin Yang finished third on seven-under after carding a 72 in a hugely impressive display from the 17-year-old, who never had a practice round at Kingston Heath.
Major winner Green finished equal fourth with compatriot Grace Kim a shot further back as Australia's hopes of a first home winner in the national women's Open since 2014 were dashed for another year.
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