Allenby, who has never lost a playoff in eight attempts, birdied the first extra hole along with Adam Scott after Craig Parry and Jarrod Moseley dropped out with pars in the all-Australian playoff.
Scott's tee shot on the 18th -- the second time it had been used in the playoff -- went into a fairway bunker. His 7-iron left him with a long putt for a birdie while Allenby's 9-iron approach left him with a 3-foot birdie putt.
The four golfers finished at 11-under-par 277 after 72 holes. Scott shot a final-round 8-under 64 to make the playoff with overnight leader Allenby (71), Parry (67) and Moseley (69).
Allenby, who broke a two-year winless drought, was weakened by a cold and flu during the tournament.
Robert Allenby has never lost a playoff in eight attempts. (Getty Images)
'It seems like I'm all right when it gets down to finishing things off,' Allenby said.
The victory recalled his performance two weeks when he took The Presidents Cup to extra holes by beating Davis Love III in the final match.
This was the second straight year Scott had been beaten in a playoff at the Masters. Last year he lost to Peter Lonard.
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