The Aussie has won every play-off as a professional after picking up victory at Huntingdale, Melbourne, following a four-way shoot-out.
Tied alongside Allenby on 11 under at the end of the four scheduled rounds were Adam Scott, three-time winner Craig Parry and Jarrod Moseley.
While Parry and Moseley were eliminated at the first extra hole, Scott, who finished with a brilliant final-round eight-under score of 64, was not so easy for Allenby to shake off.
The youngster, one of the best rising talents on the tour, sunk a testing six-yard birdie to keep in touch with Allenby, who had just holed a superb 30-footer to ensure the end of the competition for Parry and Moseley.
Allenby and Scott then returned to the 18th tee for the second extra hole, which went the way of the veteran.
While Scott struggled to get good green position Allenby pitched his second shot to within a few feet and that was that.