It was a wild 15-minute qualifying session for the Supercars field on the recently resurfaced track with times continuously improving.
The 1m19s barrier was broken for the first time, with five drivers dipping into the 1m18s.
The best of them was Mostert who bagged both a new unofficial lap record and provisional pole for Race 1 with a 1m18.676s.
That left the Walkinshaw Andretti United driver just over a tenth clear of Anton De Pasquale and means he will run last in the Top 10 Shootout that will determine pole tomorrow.
"It's pretty emotional," said Mostert. "It's the last weekend for Holden.
"Any time you can top a session in Supercars is always exhilarating, I don't remember the last time we did it in qualifying, it's been a little while.
"I'm just proud of the team, we really chased the car from first practice yesterday arvo. We stuck to our guns, kept making the car better and better, and it was fast enough to top that session.
"I'm really proud of everyone's hard work."
Scott Pye was third fastest for Team 18 while James Courtney led the way for Tickford in fourth.
His teammate Cam Waters was next, and last of the drivers to crack a 1m18s, while David Reynolds, James Golding, Andre Heimgartner, Todd Hazelwood and Thomas Randle all booked spots in tomorrow's Shootout.
The shock of the session was 2022 series champion van Gisbergen, who will start tomorrow's opening race from dead last thanks to a mistake.
The Kiwi had been set to be the first driver to dip into the 1m18s when he went purple in Sector 1 and green in Sector 2 on his final run.
However he looked to lock his rear brakes into the final corner and ended up off the road.
He got going again but didn't have time to put a flying lap together, which left him 25th and last on the grid.
The Supercars field will take part in a third practice session at 11am local time tomorrow before the Top 10 Shootout determines the first five rows of the grid at 12:35pm.
The first leg of the Adelaide 500 will then kick off at 3:15pm.
