West Australian Doull led by two shots with two holes to play in regulation, only to let John back into the contest with a double bogey on the 17th hole on Sunday at Rosebud Country Club on Victoria's Mornington Peninsula.
The pair finished 72 holes tied for the lead at 12 under, a shot clear of rising amateur star Billy Dowling (70) and fellow Australian Austin Bautista (66).
After shooting a sizzling eight-under 62 on Saturday, Doull closed with a 72 and John carded a level-par 70.
The sudden-death playoff swung backwards and forwards on the par-four 18th before Doull finally claimed the title at the fourth extra hole.
He drilled a seven-iron to eight feet and holed the birdie putt, while John, from Victoria, missed to the left from closer range.
John, 26, has been in red-hot form, winning the Queensland PGA in late November and last week's Vic Open at 13th Beach.
He sits atop the 2025-26 Australasian Order of Merit and is in the box seat to claim the full DP World Tour card on offer to the overall winner.
But it was Doull's day on Sunday after he began the final round with a two-shot lead.
"It was wild," he said after securing his second career win on the Australasian Tour.
"The double bogey at 17 definitely hurt and then I made a good putt at 18 to force the playoff.
"No-one seemed to want to quit and I just thought if we kept making pars then one of us will make bogey. But that wasn't the case at the end."
