A HORROR final round by Coffs Harbour's number one golfing export Paul Sheehan brought undone three days of good work.
Sheehan, 34, had moved to four-under after Saturday's third round at the Australian Open and was in a tie for 15th position.
Unfortunately, a last-round 79 sent him plummeting down the leader board.
Playing alongside Fred Couples, Sheehan bogeyed four of the opening 10 holes on Sunday to drop out of contention.
After a birdie on the par-five 14th hole, Sheehan then dropped another four shots on the closing four holes.
The former world number one was unable to bounce back from his third-round 75.
Chalmers hit a fourth-round 69 to secure his second Australian Open title.
Maintaining his composure, Chalmers avoided a play-off with John Senden after the Queenslander missed his putt on the final green.
Chalmers was emotional after the win, thanking his family and caddy of 22 years Mal Baker.