Traffic was critical across the two all-in sessions as drivers fought tooth-and-nail to get a tow.
The first of the 15-minute sessions came to a remarkable conclusion with the majority of the field trundling around at low speed with nobody willing to go first.
Eventually the field did burst to life, Kostecki edging Broc Feeney by 0.002s as the times tumbled.
The search for a tow got particularly tasty for Cam Waters, who tucked in behind Anton De Pasquale and went purple in the first sector.
However the two Ford drivers came to blows at turn 6, Waters bumping De Pasquale wide as both failed to improve.
That left Waters 10th and De Pasquale 19th on the grid.
Back at the top, race 1 winner Will Brown qualified third for race 2 ahead of Matt Stone Racing pair Jack Le Brocq and Cam Hill.
James Golding and Will Davison were next ahead of Thomas Randle and David Reynolds.
Shane van Gisbergen, meanwhile, was just 11th, one of his earlier runs including an off-track excursion at turn 6.
The second session followed a similar script, Kostecki once again unstoppable as he dragged the benchmark down to a 50.621s.
This time it was teammate Brown that was second as Erebus locked out the front row.
Waters ended up third after once again clashing with a DJR Mustang while trying to get a tow. This time it was Davison, the pair making contact at turn 6 after Waters had gone fastest in the first and second sectors.
Le Brocq was fourth ahead of Tim Slade, Feeney and Hill, while Randle ran earlier and was shuffled back to eighth.
Van Gisbergen was ninth and Winterbottom 10th.
The two races kick off at 1:05pm and 3:50pm local time.
