Smyth birdied his final two holes of regulation at the Royal Auckland and Grange GC on Sunday to sign for a five-under 67 and a four-round total of 15 under at the co-sanctioned $1.2 million event.
It left him tied at the top with countryman Jack Thompson, who shot the lowest final round of 64.
With darkness falling, it needed a further six nail-biting playoff holes to separate the pair, as Smyth finally triumphed with a tap-in birdie.
The 31-year-old has spent the last few years on the Asian Tour.
This victory earns him a card on the Japanese Tour and he will also get full playing rights on the DP World Tour in 2027 if he can hold on to top spot on the Australasian Tour OOM with only two smaller events to play on Victoria's Mornington Peninsula later this month.
"Japan's like one of my favourite countries to visit and play golf in," Smyth said.
"I'll definitely play whatever tournaments I can. I'm very much looking forward to that."
Smyth has had 21 top-10 finishes since his previous pro win at the Yeangdar TPC in Taiwan on the Asian Tour in 2022.
He decided earlier this year to rejoin the Australasian Tour, with three top-five finishes in a row in New Zealand, capped by Sunday's title triumph, lifting him to top spot on the OOM.
"I'm just so happy and over the moon that all the hard work, all the consistency, it's finally paid off," Smyth said.
"I've had so many top-fives and top-10s over the last four or five years and I'm watching guys win tournaments and not play as consistent as I am.
"And I'm just thinking 'when's my time going to come?' It was this week."
Justin De Los Santos from the Philippines (68) finished one shot shy of the playoff in outright third at 14 under, with Australian Jay Mackenzie (69) and Ryosuke Kinoshita (68) from Japan tied for fourth at 13 under.
Overnight leader Ryan Peake came up short in his bid to win a second big tournament in NZ in the space of 12 months.
The burly West Australian - who won the 2025 NZ Open in headline-grabbing fashion - had a final round of one-over 73 to slip back into a tie for sixth at 12 under with countrymen Jake McLeod and James Marchesani, Kiwi star Kazuma Kobori and Japan's Aguri Iwasaki.
The remaining two events on the 2025-26 Australasian PGA Tour are The Heritage Classic and The National Tournament.
