On a final day of fluctuating fortunes Allenby overturned playing partner Retief Goosen's two-shot overnight lead to join defending champion Henrik Stenson at the top of the leaderboard on 11-under-par before prevailing in a nervy play-off to become the first Australian to take the title.
Allenby squandered a chance to win it outright on the 72nd hole of regulation play when he failed to make the required par four after pushing his second far to the right, then pitching too strongly before putting too weakly.
The tall Australian from Melbourne had a putt to take the money and run but staggeringly left it short - thus setting up a sudden-death play-off against Stenson, a man who had himself three-putted the 18th to miss posting a target of 12 under.
This meant the two of them had to go back to the 18th to try to settle it in a format that required the last hole of the Gary Player course to be played three times and if a winner did not emerge to then start at the 16th until someone cracked.
But before that juncture was reached the 11 remaining players - after Rory McIlroy had withdrawn - produced a final round that was as dramatic for blotches as round three had been for birdies.
Goosen must have felt like hurling his "space shuttle" putter into orbit as he missed putt after putt, eventually again falling foul of his 2009 malady - a poor last round - and sliding away to a disappointing three-over 75.
Goosen made a miracle one-over six at the 14th after a wild hook into the bushes, the ball going even deeper in after hitting the cart path, a futile hack to get it out, a penalty drop and then a blind fifth shot that actually touched the hole but Allenby's birdie after a superb second provided the crucial swing in fortunes.
Tim Clark and Ross Fisher had their chances to possibly be part of the play-off but just could not light up the leaderboard with the required red numbers and the only crimson in Angel Cabrera's game was the colour of his neck as his temper boiled and his score ballooned.
So sudden-death it was with Allenby and Stenson the last men standing.
They would end up having to go back to the 18th tee three times before Allenby, who missed his national open to travel to South Africa, accomplished a feat that Australian major winners such as Greg Norman, David Graham and Steve Elkington could not pull off before him.
At the first time of asking and again on the second the balance of probabilities favoured Stenson as he played the hole more solidly but he was never really in a position to make a birdie three while Allenby was able to scramble his par fours.
The third time they went back it was finally Stenson who cracked as he struck a second shot that was much too strong - his ball skipping into the stand at the back of the green.
Allenby charged through the open door with a superb 6-iron approach to just over a metre and when Stenson could not save his par with an impossible chip from a scruffy lie to a glassy green the Australian could two-putt to register his first win since taking his country's Open, PGA and Masters in 2005.
In the end Allenby was a deserving winner as he was far and away the most consistent over the four days with only six dropped shots (two in his last four holes) and an inordinate number of putts that skimmed the hole - plus he didn't have a 6 on his card on Sunday!
Both Goosen and Stenson, succumbing to a sudden rash of balls splashing into the water at the 9th (Luke Donald, Nick Watney, Angel Cabrera and Ross Fisher also went fishing), made sixes as they hit the turn and then ran up one-overs at the long 14th after wayward second shots.
Stenson went way right - so far right that he hit a provisional - and Goosen miles left and in the final countdown the 6s they ran up cost them dear.
Allenby improved his play-off record to an amazing 11 out of 12 and finally found himself being able to take two putts to win - a luxury he was able to gratefully accept, quipping: "When you get two putts you might as well make the most of it!"
Source - Nedbank Challenge