It was initially a dominant showing from the #25 Walkinshaw Andretti United car, Lee Holdsworth recovering from a slightly sluggish start by barging past Tony D’Alberto in the #11 Shell Ford at the Chase on Lap 1 to grab the lead.
He then controlled the first stint until the safety car made its first appearance on Lap 17 when Thomas Randle put the #44 Tickford Mustang into the wall at Hell Corner.
That prompted a wave of first stops, Holdsworth continuing for a second in the lead car.
It was an impressive effort too as he put around six seconds over Triple Eight driver Garth Tander before handing over to Mostert on Lap 37.
Triple Eight ran the #888 longer in the second stint, Shane van Gisbergen taking over from Tander on Lap 40.
That dropped the car behind Cam Waters, who was making up time after co-driver James Moffat went off at the Chase in the first stint, and Brodie Kostecki who had pitted out of sync for new brake rotors to remedy a long brake pedal.
On Lap 49 there was a scare for the #25, the left-rear tyre delaminating as Mostert came across Skyline.
He limped back to the pits for an unscheduled stop, resuming in 12th before charging back into contention with a stirring drive across the fourth stint.
Waters, van Gisbergen and Kostecki all handed their cars back to their co-drivers for the fourth stint.
Moffat continued to run second behind the out-of-sync Mostert followed by Tander and, initially, David Russell.
Russell later dropped another spot when he was overhauled by Anton De Pasquale, who was running against the co-drivers in the #11.
On Lap 60 Mostert pitted from the lead to hand his car back to Holdsworth, who resumed in sixth place.
He then filtered back to the top as the likes of De Pasquale (Lap 76), Moffat (Lap 77) and Tander (Lap 83) all took stops.
As it stands Holdsworth leads Moffat and Tander.
There was heartbreak early in the race for the #9 Erebus Motorsport entry, the car taken out of contention 30 laps in with a power steering failure.
Jack Perkins was sitting eighth when the system let go.
“It’s obviously had a power steering failure,” said primary driver Will Brown. “It’s disappointing because we’ll just be circulating for the rest of the day. Not much you can do. That’s part of it.”
The car has since got going again but is more than 14 laps down.
The Scott Pye/James Golding entry suffered a power steering failure even earlier the race, efforts to rectify it proving unsuccessful.