Mostert has been deemed guilty of careless driving in an opening lap incident with PremiAir Racing’s James Golding.
The WAU Mustang driver attempted to overtake Erebus rookie Cooper Murray at Turn 11, only to lock an inside front and cannon into Golding’s Camaro.
Contact spun Golding into the gravel trap, triggering the first of two Safety Car periods for the race.
Mostert was able to continue with sizeable damage to the left-front corner of his Mustang, shadowing teammate Ryan Wood to the chequered flag.
Officials have now slugged his a 30-second time penalty for his troubles, dropping him to 22nd.
That position could yet change again, however, pending other outstanding investigations.
It capped off a troubled day for Mostert, who will start Sunday’s finale from 23rd after being impeded by Will Brown on his final flying lap.
