The veteran inherited the lead early in the race thanks to a spectacular engine fire for Cam Waters, before withstanding intense, race-long pressure from Broc Feeney.
The win was Winterbottom's first since Pukekohe 2016 and first podium since Wanneroo in 2018.
It is also Team 18's first win in Supercars.
"I'm emotional," said Winterbottom. "That's awesome. I told [team owner] Charlie [Schwerkolt] when I joined that we were going to try and win, and far out, it's taken a while.
"That was hard work, that was really hard work. That's awesome, I'm pumped."
Waters made a solid start from pole, using the inside line to run Broc Feeney wide around the long first corner.
That opened the door for Winterbottom to sneak past Feeney and grab second place.
Source: Autosport