The Dick Johnson Racing Mustang driver qualified fifth and ran as high as fourth early before slumping to an eventual 12th.
That was due in part to a refuelling issue at his first stop that left him to fuel-save throughout the second stint.
Kostecki, however, was also heard complaining about the car’s handling via team radio and later said he was confused inside the car.
“I think I was just pretty confused to be honest, inside the car, I knew straight away five laps in that I was in a world of hurt,” he told the broadcast
“Honestly, before I had the tangle with Davey [Reynolds] yesterday I felt really strong in both races, just didn’t sort of maximise qualifying to be honest.
“We fixed qualifying today, but funnily enough during qualifying I was quite unhappy with the car, and it was doing things it shouldn’t have been doing.
“I think we were just lucky enough to fluke a lap out of it.
“The pace really shows in the race when things aren’t quite right and you’re doing laps consecutively.
“We’ll go back to the workshop and sort it out, you know, Darwin’s a new round.
“We come to another track and the car was fast, yeah we didn’t finish where we wanted today, but still proud of the effort.”
Engineering George Commins had an answer for the issue in the pit stop but not the mysterious lack of speed.
“We had a vapour lock or something in the fuel system and didn’t get hardly any in during the first stop, which puts you massively on the back foot for the rest of the race,” he said.
“We could still do it on two full churns but we needed a bit of fuel saving in the second stint from Brodie which he did easily, and then it all balanced out.
“It’s something we have to look into after this, the multitude of errors in this race.”
As for the handling issue, he added: “It’s something we have to look at when we get back.
“Obviously we went from being on provisional pole to being one of the slowest cars here today.
“Something has obviously gone wrong, we’re going to have to look through it all and find the problems.”
Teammate Rylan Gray finished 23rd after having to complete a fourth pit stop in the closing laps.
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