'I played good out there all day,' said Sheehan, who collected his first Nationwide Tour victory. 'This feels awesome. It just happened to be my turn to win.'
Sheehan posted a 3-under 69 on Sunday at Royal Adelaide Golf Club to get in the clubhouse at 7-under-par 281. Sim, who held the 54-hole lead, carded a 1-under 71 to force the extra session.
The first playoff hole, which was contested at the 18th, was not the picture of spectacular golf. Neither player found the fairway, with both coming to rest in the right rough.
Sheehan knocked his second in a greenside bunker, while Sim flew over the green. Both recovered to make pars, but the driver would haunt Sim as they played 18 once again.
Sim's drive at 18 sailed right and landed out of bounds between a tent and a trash can. He was forced to take a penalty, opening the door for Sheehan, a former member of the Japan Golf Tour.
Sheehan did not play the hole very well either, but when Sim missed a 15-footer for bogey, the opportunity was there for Sheehan. He had two putts from 3 feet and closed the door with his bogey.
'To win in a playoff feels fantastic,' said Sheehan. 'You know so much is on the line. You've played great, but you haven't finished. You have to buckle down.'
Sim, who turned professional in November of 2005, took the second-place finish in stride.
'I had a lot of opportunities,' said Sim, who came up just short on a chance to win on the 72nd hole. 'I was pretty disappointed when I found out it was out of bounds. I just hit a terrible shot, but that is life.'
Nick Flanagan shot a 1-under 71 to finish alone in third place at minus-6.
Aron Price (70) and Jarrod Lyle (71) shared fourth place at 5-under-par 283.
Craig Bowden fired a five-under 67 on Sunday and tied for sixth with Peter Senior, who posted a 1-under 71 in the final round. The duo came in at 4-under-par 284.