The 22-year-old from Victoria stormed home on Sunday with a five-under par final round of 67 to win by one shot.
He went one better than Jason Day, who had gone the closest as an amateur to claiming the title in the event's 97-year history when runner-up in 2005.
Campbell, who has been in the Golf Australia high performance squad for the past two years, can now turn professional because of his win.
"I'm definitely leaning towards turning pro," said Campbell, who won the Riversdale Cup in Melbourne earlier this year.
"With winner's category, now it's definitely heading that way.
"I will talk to my team, see what they say, what they recommend and we'll take it from there."
Campbell started the final round eight shots off the pace at Brisbane's Kurrai course, but overnight leader Andrew Kelly faded on the back nine.
A three-foot putt from the back of the 18th green gave Campbell a tournament total of nine-under 279.
He finished one shot clear of seven others, with reigning Challenger PGA Tour of Australasia order of merit winner David Micheluzzi from Victoria just missing a 10-foot birdie putt on the last that would have tied him with Alexander.
Rookie Harrison Crowe from NSW also finished joint-second after joining New Zealander Kazuma Kobori with Sunday's best round of 66.
Queenslanders Blake Proverbs, Deyen Lawson and Lawry Flynn, NSW's Jack Pountney and New Zealander Nick Voke were the other joint runners-up to win $18,085 apiece.
Kobori finished on 281 for joint ninth overall.
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