British Open champion Smith's 14-under-par total, courtesy of a three-under-par final round, secured him victory at the rain-hit A$2 million ($1.35 million) event co-sanctioned by Europe's DP World Tour.
The 29-year-old, back in Australia after nearly three years blighted by the COVID-19 pandemic, previously won back-to-back titles at the event in 2017 and 2018.
"It's awesome. I really didn't think I had it in me at the start of the week," Smith said.
"I was a bit scratchy and the game has got better and better as the week went on, other than the front nine today."
When asked about the two weather-enforced delays in play, Smith said, "you are in the mojo a little and for it to be stopped not once but twice was a little frustrating but I just held on and played really solid those last eight holes."
An emotional Smith paid tribute to his grandmother, who walked all 72 holes with him as a spectator despite having recently undergone chemotherapy.
"I can't believe she did it," Smith said. "Everyone at the start of the week was telling her to pace herself. It's pretty amazing and definitely inspiring."
Hisatsune and Scrivener were hot on overnight leader Smith's heels towards the midway point of the final round, but were unable to keep up their momentum and eventually faded away to finish in joint second.
Englishman John Parry and Australia's Min Woo Lee finished tied fourth, while two-times winner Adam Scott carded a three-under 68 to end tied for 28th.
Smith will next be in action at the Australian Open, the regional circuit's most prestigious event, which returns in Melbourne next week as an integrated men's and women's tournament after being cancelled twice during the pandemic.
