MELBOURNE, 23 Nov 2014 (AFP) - World number two Scott was trying for a third successive Australian Masters title but fell agonisingly short and was forced to settle for a three-way share of second place after shooting a 4-under-par 68.
Cullen's lead looked in danger on the final hole but a brilliant bunker shot to within a foot of the hole helped him save par.
Scott had three birdies on the back nine while another birdie putt that could have brought him level with Cullen on the 18th lipped out.
Newcastle's James Nitties and Victorian Josh Younger were tied with Scott for second.
Cullen, 30, is the twin brother of former Test cricketer Dan.
Ranked 539 in the world, his previous biggest win was the 2012 Indonesian Open on the OneAsia tour while he won the Queensland Open in 2013.
He led on the opening day of the Australian Masters last year and finished fourth overall behind Scott.
Cullen said he could not recall ever making a more valuable bunker shot than on the 18th.
"It would have been nice to have a little buffer coming down here but a couple of three putts gave that away," he said.
"To get up and down like that and have a win it's an amazing feeling."
Leading scores after the final round of the Australian Masters at the Metropolitan course (par-72) in Melbourne on Sunday (Australia unless stated):
279 - Nick Cullen 73-71-66-69
280 - Adam Scott 73-68-71-68, Josh Younger 70-70-71-69, James Nitties 72-71-67-70
281 - Matthew Griffin 71-69-72-69, Aron Price 68-73-71-69, Adam Bland 75-68-69-69, Paul Spargo 70-67-71-73
282 - Rhein Gibson 68-74-75-65, Robert Allenby 70-73-71-68
283 - Anthony Houston 71-68-75-69, Bryden Macpherson 70-71-72-70, Matthew Guyatt 70-69-73-71, Lucas Herbert 75-70-65-73
284 - Boo Weekley (USA) 72-72-70-70, Brett Rumford 74-68-71-71
285 - David McKenzie 72-70-74-69, Todd Sinnott 67-71-77-70, Steven Bowditch 67-75-73-70, Ryan Ruffels 75-68-72-70, Tom Bond 70-72-72-71, Stephen Dartnall 72-73-69-71, Peter Wilson 70-71-72-72, Chris Campbell 72-74-66-73
Selected scores:
286 - Zac Blair (USA) 70-75-69-72
287 - Kyle Stanley (USA) 69-73-73-72
292 - Riley Wheeldon (CAN) 71-70-81-70