The 30-year-old from Melbourne led from start to finish this week in becoming the first player to win this event in back-to-back years since Greg Norman in 1984-85.
Allenby closed with a 3-under 69 for a 72-hole total of 15-under-par 273 at Royal Queensland Golf Club. The victory was his seventh on the Australasian Tour and his 15th worldwide.
Allenby began the day with a one-shot lead over fellow Australians Geoff Ogilvy and Craig Parry. But while Parry fell by the wayside with an early double-bogey, Ogilvy was able to wrestle the lead away from Allenby with a pair of birdies through the first six holes.
Allenby then fell two shots back of the 24-year-old Ogilvy with a bogey at the seventh hole and made the turn at 11-under.
Ogilvy dropped to 12-under after bogeying the 11th and Allenby pulled even with a birdie at the next hole. Allenby came up big at the par-5 13th, holing a bunker shot for eagle to regain the lead at 14-under par.
Although Ogilvy, who birdied No. 13, slipped two off the pace with a bogey at the 14th, a two-shot swing at the 15th -- an Ogilvy birdie against an Allenby bogey -- leveled the competitors at 13-under.
But the more experienced Allenby tallied birdies at Nos. 16 and 17 to move two shots clear of his playing partner. When Ogilvy rolled in a 35-foot birdie to close within a stroke at the 18th, Allenby responded with the clutch par save to take the title for a second straight year.
'I knew it was going in,' said Allenby of his victory clinching putt. 'I knew if I put it on line there was no chance it was going to miss.'
Had Allenby missed the putt he still would have been favored to win. He owns a career playoff record of 7-0 and has earned three of his four U.S. PGA Tour victories in extra holes.
Ogilvy, who is still seeking his first win, shot 69 to finish alone in second place at 14-under 274.
Gareth Paddison, a New Zealander making his debut as a professional this week, had a share of the top spot earlier in the day but wound up with a 69 of his own to tie for third with Parry at minus-11.
Parry, who set the tournament record of 19-under-par 269 when he captured the Australian PGA in 1992, carded an even-par 72 in the final round.
Mathew Goggin finished solo fifth at 10-under after a 71, while Norman fired the day's best round of 6-under 66 to climb into sixth place at 9-under.
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