Scotland's David Law eagled the 18th as he overcame a three-shot deficit in the final three holes to win the ISPS Handa Vic Open - a first European Tour title.
Law, who began the day three shots off overnight leader Ormsby, struck a brilliant approach to within 10 feet of the pin on the par-five 18th and coolly rolled in the eagle putt to pressure the leading Ormsby.
Law: "It's not something I expected".
The 25-year-old from Montrouge, France, returned an even-par 72 in Sunday's final round to finish at -8 and become the season's first Rolex First-Time Winner.
The Australian also missed a long eagle putt on the last that would have forced a playoff.
Celine Boutier from France won the women's competition, with England's Charlotte Thomas joint second.
He ended on 18-under-par after a round of 66, winning his first European Tour event after only securing his playing card last November following a stint on the Challenge Tour.
Overnight leader Ormsby still looked the likely victor until he made a mess of the par-3 17th hole, missing the green to the left and taking two chips before walking away with a double-bogey.
Scot David Law won the men's event for his first European Tour success.
The pioneering tournament, which is co-sanctioned by the European Tour and the LPGA Tour, is the only one in the world with male and female fields, playing in alternate groups on the same course and for equal prize money.
Fellow Australian Kennedy (67) tied for second alongside Ormsby on 17-under 271 in the $1.5 million tournament, with South African Justin Harding a shot further back in outright fourth.
"I was kind of struggling a little bit in the beginning but I just held on and kept fighting to the end". I'll go home, reflect a little bit on things and reassess.
"We got hammered with the rain for about three or four holes there, so it was really good to get through that stretch", Asian Tour regular Stieger said after sinking seven birdies to stay in the hunt.
"We stood on the tee and we said we need to make eagle", Law said during the trophy presentation. "I'm super excited with the way I handled myself". "It's been something I've been working towards since I turned pro and I just feel like it just happened -- I don't think I realise right now".
He tweeted: "The work we do @paullawriefound is not to raise golfers to win on tour but when one of our kids does win on tour which happened today it's an unbelievable feeling".