The 29-year-old Presnell finished at 9-under 279 on the Moonah Links course in the event sanctioned by the Australasian and Nationwide tours. He earned $108,000 and full Nationwide Tour status for the rest of the season.
O'Malley bogeyed the 17th en route to a 70. Australia's Michael Sim (70) was third, three strokes behind. Americans Skip Kendall (70) and Daniel Summerhays (72) finished at 5 under along with Australians Terry Pilkadaris (70) and Adam Bland (71).
Argentina's Miguel Angel Carballo, two strokes ahead after the third round, bogeyed three holes on the front nine and shot a 78 to tie for 13th, six shots behind Presnell.
The 29-year-old Presnell will receive full playing rights on the Nationwide Tour this year. He said without a top finish in this and two following tournaments in New Zealand, he would have quit golf and returned to the air conditioning industry.
"I was going into these four events (including last week's Johnnie Walker Classic) thinking I could try to get some money together to do a trip as I did last year, drive across America and do some Monday qualifiers. ... If not, then I'd have to find a job," Presnell said.
"These four weeks were pretty much about it. I did an apprenticeship in air conditioning so I might have to go ask the boss for my job back there or do something else."
Presnell said he changed his training routine.
"I'm just a bit more diligent with my practice instead of standing there just beating balls," Presnell said. "It got back into my mind why I am practicing and what I need to practice."
O'Malley, while disappointed with his finish, said his play gave him the confidence with his game from tee to green to attack the European Tour this season with renewed optimism.
"I missed a few short putts today but everything feels pretty good," O'Malley said. "The way I'm playing this is a huge boost. Last year I just wasn't quite playing well."