Where the majority of the field set quick times in the cooler morning conditions, Mostert dialled in his #25 entry and set his quickest lap, a 1:20.0356s, in the car's 86th lap late in the day.
Speaking prior to the final runs, Mostert told Supercars.com that the focus was on making speed improvements, albeit without a single-minded view on the fastest time of the day.
"Plenty of laps, the weather was on down here in Victoria," he said.
"We're just trying to go through our programme and see how we end up at the end of the day.
"Today's all about programming and doing little changes.
"The track condition is only as current as that hour.
"It changes so much from the cooler track temp this morning to where we are now in the day.
"But you're only as good as your last run. You want to keep trying to make improvements through the day."
Temperatures rose to 31 degrees with two hours remaining, with teams completing large testing programmes.