GOLD COAST, 14 Dec 2014 (AFP) - Scott had four opportunities to successfully defend his title with birdie putts during the marathon playoff at Royal Pines but failed to convert them.
When he then missed a four-foot putt for par on the seventh playoff hole, it allowed Dallas-based Chalmers to seal the win with a near tap-in.
"Just phenomenal, I'm worn out," two-time Australian Open champion Chalmers said.
"I was all over the place... really excited and thrilled."
The playoff, the longest in history for a tier one tournament in Australia, started with three players after Scott, Chalmers and another Australian Wade Ormsby finished level on 11-under.
Left-hander Chalmers won his first Australian PGA Championship in 2011, also following a three-way playoff.
Ormsby forced his way into the sudden death shoot-out with a long putt on the 18th hole to complete a one-under 71 fourth round.
He had a brilliant chance to win the title on the second playoff hole but missed a putt of just four and a half feet.
On the third playoff hole, Ormsby's brave bid for a second professional win faded as he went wide on his approach and his rivals both made birdie.
Chalmers's putting proved the difference after he had nailed eight birdies during his final round to post a tournament low eight-under-par 64 to surge to the top of the leaderboard.
Scott regularly failed to sink birdie putts and was made to pay for his profligacy. But he said it was his approach shots to the green that weren't good enough.
"I didn't hit it close enough today to the hole," he said.
"It wasn't like I missed 10 footers today all day long.
"When you hit it outside 25 feet, there is almost the same chance you are going to three putt as two putt on tour. You have to hit it closer," he added.
Leading scores after the final round of the Australian PGA Championship at Royal Pines (par-72) on the Gold Coast on Sunday (Australia unless stated):
277 - Greg Chalmers 71-71-71-64, Wade Ormsby 68-67-71-71, Adam Scott 68-69-69-71 (Chalmers won at 7th playoff hole)
279 - Michael Hendry (NZL) 70-71-67-71
280 - Scott Stallings (USA) 72-70-68-70
281 - Boo Weekley (USA) 66-72-69-74
282 - Matthew Giles 73-69-70-70, Scott Strange 69-66-71-76
283 - John Senden 73-68-73-69, Cameron Smith 74-68-71-70
284 - Marc Leishman 69-71-74-70, Ryan Fox (NZL) 70-74-69-71
285 - Brad Kennedy 69-73-75-68, Robert Allenby 72-69-72-72, Geoff Drakeford 73-68-72-72, Brendon De Jonge (ZIM) 71-71-70-73, David Bransdon 73-70-68-74
286 - Peter Lonard 70-74-72-70, Matthew Guyatt 72-72-70-72, Jun Seok Lee 73-68-72-73, Michael Wright 71-73-69-73, Brett Rumford 72-71-70-73, David Smail (NZL) 68-71-71-76, Jason Scrivener 73-66-68-79
Selected scores:
288 - Jin Jeong (KOR) 65-74-77-72, Ryan C Carter (USA) 73-71-71-73
289 - Conrad Shindler (USA) 72-72-74-71
290 - Garrett Sapp (USA) 75-68-75-72
291 - Rory Hie (INA) 71-71-69-80
294 - John Young Kim (USA) 72-71-76-75
297 - Ratchapol Jantavara (THA) 69-72-76-80