The Penske driver – who came all the way from Q1 – had set a benchmark of 1m20.891s in tricky, slightly damp conditions, Mostert responding with a 1m20.622s right at the flag.
“This is for all the boys and girls at Tickford,” Mostert said.
“It’s been a bit of an up and down year for us and these poles don’t come easily.”
It could have been a three-way fight for pole, Scott McLaughlin having gone purple in the first sector of his final lap. However he ran wide of the dry line heading into Turn 7, which meant he didn’t improve on his best time, and slipped back to fifth.
That was good news for David Reynolds, who also went through Q1, and James Courtney, the pair slotting into third and fourth.
Mark Winterbottom will start sixth, Scott Pye capping off a promising session for Walkinshaw Andretti United with seventh.
There was a pair of surprise Top 10 participants next, James Golding and Garry Jacobson eighth and ninth.
It was a quiet session for Triple Eight, Jamie Whincup the best of the Red Bull Holdens in 10th. His teammate Shane van Gisbergen didn’t even make it into Q2, the Kiwi set to start today’s race from 12th.
Cam Waters will start 11th, as the unlucky man on the Q1/Q2 bubble. The Tickford Mustang driver was bumped out of the Top 10 by Reynolds during a frantic end to the middle segment.
Earlier in the session there were some big names that didn’t make it out of Q1, including Mustang-shod Will Davison, who will start way down in 16th.
Brad Jones Racing pair Nick Percat and Tim Slade couldn’t get beyond the first segment either, Percat left stranded in 17th while Slade will start from 19th.