"It all came down to strategy," said Mostert, 23. "Obviously the rain played a big part and the car was pretty good in the rain. A big thanks to the team."
Holden veteran Craig Lowndes, who came into the weekend hot on Winterbottom's tail but is now third overall, came in 10th. Teammate and six-time champion Jamie Whincup couldn't build on his drought-breaking win and podium on Saturday, managing only 14th place.
Volvo's Scott McLaughlin started on pole but had to tussle with the Ford of David Reynolds for the lead on the opening lap.
McLaughlin held on, but his effort was undone by a safety car just as the second lap began - triggered by a collision between his teammate David Wall and Nissan's Todd Kelly.
The entire field headed straight for the pitlane to top up with fuel and mix up their tyres.
Mostert came out in front, ahead of Reynolds, as each team's strategies began to play out.
But they all went out the window when the heavens opened on lap 25.
Dale Wood was the first casualty, sliding his Commodore on the wet track and beaching it in the gravel.
It prompted another safety car and a slew of manic pitstops.
Mostert again emerged in front and stayed there for the rest of the race, cut by two laps due to broadcast and scheduling time restrictions.
The championship next heads to Sandown from September 11, marking the beginning of the endurance series.
The Bathurst 1000 and Gold Coast 600 follow on October 11 and 25.
James Courtney looks set to miss next month's event at Sandown, as the Holden star continues to recover from chest injuries suffered in a freak accident in pit lane. He was hit by a metal hoarding blown over by a navy helicopter during a display.
- AAP
