Prior to the 200km race, the #1 Toyota Supra had been mired in the midfield for the entire Christchurch Super440 weekend.
Mostert qualified 10th or worse for each of the four races and ultimately ended up with results of eighth, 12th, 12th and 17th.
So the fact Mostert was holding down fourth when he got stuck into it with Kostecki on the penultimate lap was a surprise in itself.
Mostert had also lagged behind teammate Ryan Wood at Taupo until shooting into contention during the 200km race, where he scored his only podium of the year to date.
The reigning champion described the NZ swing as “two tough weeks”, noting the Gen3 cars are “super fiddly” and Ruapuna required “almost a perfect car to extract speed”.
“The margins are so small with the car,” Mostert told Speedcafe in the wake of the Christchurch finale.
“You’ve got only a certain amount of things you can play with in Gen3 and all little measurements here and there mean the world.
“So even though you think you might sometimes [understand] exactly where you are, you might just be a little bit out of that window.
“With this characteristic of track, I just never really found it untill the end of the weekend.
“It was good to at least just have one race where I seemed to have some pace, [but] we need to try and roll out a little bit better on that side of things.”
Mostert had spoken after the final race at Taupo about the performance window being so small that the balance – and therefore speed – shifted greatly stint to stint.
“For us a bit older boys, it’s really a bit like sim, you’ve just got to hit your marks and believe that the car is going to do this,” he said.
Teammate Wood has been able to extract speed out of the new Supra more consistently than Mostert in the early rounds.
Wood’s competitiveness, though, notably fell away on Sunday before being struck down with a failure.
Mostert expressed empathy for Wood in the aftermath of the Jason Richards Trophy slipping through the youngster’s fingers.
“I’m just gutted for him,” said Mostert.
“We’re all one team and we’re all pushing as far forward as we can. He should be so proud of what he’s done over these two weeks.
“Like anything, this hurts as a team, but we saw what happened last year when we had some things that didn’t go our way. We use it.
“Our team’s pretty special, we use that pain as a strength and motivation to push forward.”
Mostert sits 11th in points after four rounds, while teammate Wood is sixth.
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