Friday's track action kicked off with the first of four 40-minute practice sessions, and one of just two open to the Pro drivers in the field.
The times tumbled quickly in the early part of the session, the first representative lap, a 2m05.235s, coming from Maro Engel in the #999 GruppeM Mercedes inside the first 10 minutes.
Fellow Mercedes driver Broc Feeney eclipsed that time in the #888 Triple Eight entry with a 2m05.136s, before former winner Matt Campbell became the first driver into the fours in the Manthey EMA Porsche with a 2m04.549s.
Just after the quarter hour mark the benchmark was lowered again, this time by Frederic Vervisch in the #55 Schumacher Audi who went quickest with a 2m04.350s.
That time proved tough to beat, the Fuchs-backed Audi holding on to top spot until the final two minutes.
By that point Supercars star Mostert was on a charge, steadily working his way up the order to end up on top with a 2m04.260s in the #65 Melbourne Performance Centre Audi.
That left Mostert leading Vervisch at the flag, with Tony D'Alberto a late improver as he put the #6 Wall Racing Lamborghini into third.
The Manthey Porsche ended up fourth thanks to Campbell's early lap, one spot ahead of the reigning winning entry, the #75 SunEnergy1 Mercedes.
The #99 Triple Eight Mercedes, the squad's Pro-Am entry, ended up sixth fastest as the #32 entry led the way for Team WRT BMW with the seventh fastest time.
The #777 MPC Audi was eighth ahead of the #888 T8 Mercedes and the best of the Silver entries, the #10 International Motorsports Audi.
