Mostert dropped from 81 to 189 points behind leader and race winner Will Brown with a seventh-placed finish alongside co-driver Lee Holdsworth on Sunday.
It was a result that team principal Carl Faux declared to be more than the team deserved after a mistake-riddled race.
Their issues included an uncharacteristic spin from Holdsworth and multiple strategic missteps, including not pitting for tyres under a late yellow.
WAU’s #2 Mustang of Ryan Wood and Fabian Coulthard was just 15th after an equally fraught race that included a penalty for an unsafe pit release.
“Having both cars in the Top 10 Shootout is something to be happy about but we made too many mistakes in the race,” reflected Faux post-race.
“Seventh place was more than we deserved. We just weren’t good enough.
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“The pace is there but the execution was diabolical in every area. It’s a bitter pill to swallow and we must learn from it.
“Everyone will be in the workshop at 8.30am on Monday for a full re-set.
“We’ll now look forward to one of our strengths, which is racing at Mount Panorama.”
Mostert agreed that the result was “pretty lucky” considering how the day played out.
“I kind of wanted to pit there at the end, but bit of confusion for us,” he admitted.
“So I ended up staying out and then it obviously went time certain, so I survived on a bit of a slightly older tyre than the guys around us.
“Our car was really fast in clean air, but today just wasn’t our day. Just too many mistakes, and we just didn’t want it enough today.
“If this one stings, it gets us very motivated for the next one and we’ll look forward to that.”
Holdsworth had lashed his own performance while speaking on the TV broadcast during the race.
“That was horrible. I just feel terrible for the team,” he said of his unassisted spin at the exit of Turn 1. “It was a shocking mistake.”
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