The Dick Johnson Racing star had carried a viral infection into the event and pulled the pin after waking up on Sunday morning with lower back pain.
Co-driver Todd Hazelwood took over the car for the final day of action while Kostecki underwent a precautionary scan on his back at Darwin hospital.
Kostecki had been unwell for over a week before the trip to Darwin and said he was in trouble from the moment he arrived.
“I thought I was actually getting better before I flew out on the Tuesday, but the Tuesday flight really knocked me for six,” he said on Lucky Dogs.
“I just had like a normal cold or flu leading into it, but it seemed to turn into something more sinister as I got into the weekend. It was pretty rough.”
Kostecki said he had low energy and aches early in the race weekend, but “still felt sharp upstairs” and was running on adrenaline.
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He managed to score pole on Friday and finish fourth in the race, despite “coughing flatout in the car”.
Kostecki then finished third in the 200km Saturday race but required urgent medical attention in the aftermath, skipping post-race commitments including the podium.
Put on a drip at the medical centre, Kostecki rebounded quickly and was able to complete his post-race debrief at the circuit.
“I was fine to drive the next day,” he said.
“I went back to the hotel and hopped into bed and woke up the next morning, my body just completely shut down.
“It was a task to walk, to be honest.”
Experiencing painful spasms in his lower back, Kostecki phoned engineer George Commins to tell him he was “99 percent” sure he would not be able to drive.
“I was having muscle spasms mainly towards my lower back, but other parts of my body as well, like my knees and all sorts of stuff,” he said.
“Like, I was having a combination of things that was not allowing me to basically do the job at hand and do it safely to that matter.”
Kostecki went to the track hoping physio would provide relief and, although receiving medical clearance to compete, pulled the pin and headed to Darwin hospital.
“I went to the hospital to get precautionary scans on my back, and I got the scans back and they’re like, ‘hey, with what you’re feeling, we’re not really seeing much here’,” he said.
“That was a bit of a sign and obviously got back to Brisbane later that night and went and saw a doctor, and got a bunch of tests done, and feel like we’re on top of it now, which is good.”
Kostecki has been undergoing further physio treatment this week and is confident he’ll be good to go for the next round of the championship in Townsville on July 10-12.
“I’ve got obviously a physio side of things to make sure that there’s no other underlying issue that could have caused some of the issues that I had on the Sunday,” he said.
“But it’s basically just going through a process of trying to get rid of the infection, which is still an ongoing process.”
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