Kostecki was largely untroubled in the final 20-lapper of the weekend with a challenge from Cam Waters persistent but at no point overly dangerous.
Even better for Kostecki was that his key title rivals all failed to fire, Shane van Gisbergen the best of them in fifth place.
That gives Kostecki a 137-point lead over van Gisbergen, while Broc Feeney is now 258 points back after finishing last in that race.
That was thanks to being hip-and-shouldered off the road at the last corner on the penultimate lap by David Reynolds as they fought over ninth place.
Will Brown, meanwhile, is a hefty 258 points back after a difficult day that ended with a 13th-place finish.
Kostecki made a solid start to the race from pole position, his biggest challenge coming from Nick Percat who vaulted off the second row and had a look around the outside into Turn 1.
Percat eventually had to settle for second, while Waters dropped from second to fourth, before working his way past Thomas Randle and back into third on the first lap.
Source: Autosport