Kostecki was largely untroubled in the final 20-lapper of the weekend, 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 238 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.
Third then became second on the next lap when he marched past Percat on the run into turn 1.
By that point Kostecki was a second down the road, the pair see-sawing around that margin across most of the first stint.
It wasn't until the stops neared that Kostecki was able to pull further clear, the gap out to more than two seconds when Waters stopped.
Kostecki covered Waters off the next time around, resuming with his two-second lead in tact.
From there it was a simple case of managing the gap, Kostecki cruising to the line as he scored a clean sweep by nine-tenths of a second.
"Awesome effort by everyone at Erebus," said Kostecki. "It was fantastic execution the whole weekend, fantastic pitstops. All of the effort that's gone into this year is starting to pay off.
"The car has been lightning quick all weekend. I had to hold off Cam there, he was breathing down my neck and pushing me the whole race. I'm glad to take away the clean sweep. First one for myself and first one for the team."
Randle capped off a career-best weekend with third place, while Percat scored his best result of the season so far in fourth.
Van Gisbergen was fifth to limit the damage to his title tilt with Dick Johnson Racing pair Will Davison and Anton De Pasquale sixth and seventh.
James Courtney was eighth, Will Davison ninth and Jack Le Brocq 10th.
The season continues with the first of two endurance races, the Sandown 500 on September 15-17.
