After cruising through the first two segments of qualifying, Mostert and Waters faced off in a fierce battle for pole in Q3.
It was Waters that drew first blood with a 1m08.130s on his first run, before Mostert pieced together a two-lap run in the 1m07s, the best a 1m07.630s.
Waters did his best to respond on his final run but fell just 0.04s short.
“I can’t believe it,” said Mostert.
“We were struggling in practice with the balance of the car, but credit to [engineer] Adam [Deborre] and the whole crew at Walkinshaw Andretti United, we worked really hard. The guys haven’t stopped, they haven’t had a lunch break. It’s a big reward for the team.”
Jamie Whincup led the way for Triple Eight in third place, 0.15s behind Mostert, while Mark Winterbottom kept his strong start to the weekend going with fourth on the grid.
Jack Le Brocq was impressive in his Tickford Mustang, edging Anton De Pasquale and Andre Heimgartner for fifth.
Will Davison was eighth quickest for Dick Johnson Racing, while Erebus got both cars into Q3, Brodie Kostecki and Will Brown qualifying ninth and 10th.
A number of big names didn’t feature in the final segment if qualifying, the biggest shock points leader Shane van Gisbergen. Despite nursing a broken collarbone the Kiwi comfortably ran in the Top 10 in practice, only to slump to 17th in qualifying.
“I just didn’t drive well enough and didn’t get the tyres phased good enough,” said van Gisbergen. “It was only a tenth to get in and we didn’t quite make it.”
James Courtney failed to convert his promising practice pace, the Tickford driver left on the bubble in 11th, while the Nick Percat (14th), David Reynolds (16th) and Tim Slade (19th) were among those bumped out of Q2.
One of the surprises of the session was Bryce Fullwood, the Walkinshaw Andretti United driver knocked out of Q1 and left on the back row of the grid.
He explained that his best lap was ruined by a lock-up, while the resulting flat spot left him powerless to improve on the second lap of the run.
“I was on a pretty reasonably lap there and I locked the left-front really bad into Dandenong,” he said.
“I had a severe flat spot, but I knew I had to press on. I was a bit scared the tyre was going to explode down he back straight, but I knew I had to put some form of lap in. I just couldn’t make it happen.”
Jack Smith, meanwhile, will start the race from dead last, his qualifying campaign ending in the gravel on his first lap in Q1. The subsequent red flag meant he couldn’t take part in the rest of the session.
“I just pinched a wheel, there’s a little kerb there on the left hand side, nicked it and I knew that was going to be that,” he explained.
“It was an unrecoverable lock-up. I tried to keep it going in the gravel but it’s a bit of an awkward gravel trap and I was never going to get out. It’s my fault.”