The early focus in the half-hour session was long runs with the likes of Shane van Gisbergen and Randle cycling through top spot.
The first representative time came from Scott Pye at the midway point, who set the benchmark at 1m19.445s.
In the final third of the session that was lowered to a 1m19.155s by Mostert and then a 1m19.135s by van Gisbergen.
James Courtney and Andre Heimgartner went quicker in the last few minutes before Chaz Mostert jumped to the top with a 1m18.857s.
Anton De Pasquale looked set to eclipse that time in the final minute, the Shell Ford driver going quickest in Sectors 1 and 2.
However his lap was undone by a red flag promoted by a nasty high-speed crash for Randle at Turn 8.
That left Mostert in top spot, while Tickford Racing now faces a race against time to repair Randle's car before the Top 10 Shootout in an hour.
Heimgartner ended up second for the season ahead of Courtney and van Gisbergen.
Randle's earlier time was still good enough for fifth, one spot clear of Tickford teammate Cam Waters.
Pye drifted back to seventh as the session wore on from David Reynolds, Will Brown and Macauley Jones.
De Pasquale, meanwhile, was left down in 15th after missing out on that final lap.
The Top 10 Shootout kicks off at 12:35pm local time.
