Tickford Racing's Chaz Mostert shaded Holden pair David Reynolds and Shane van Gisbergen in first practice for the BetEasy Darwin Triple Crown.
Mostert took his Supercheap Auto Mustang to a 1:06.7219s in the closing stages to edge Reynolds – atop the order for most of the 30 minutes – by 0.0640 seconds at Hidden Valley.
Van Gisbergen was third and 0.0944s adrift, his Red Bull Holden team-mate Jamie Whincup 12th, and the Shell V-Power Mustangs of Scott McLaughlin and Fabian Coulthard 11th and 16th.
All four members of that quartet had off-track excursions, including van Gisbergen and McLaughlin together at Turn 1.
Reynolds led the way after the first runs on a 1:07.0562s, with McLaughlin and Whincup both within 0.045s.
He lowered the benchmark to a 1:06.7859s at the halfway mark, and headed team-mate Anton De Pasquale in a Penrite Racing one-two as the final runs began.
Mostert claimed top spot with 90s left on the clock with a 1:06.7219s, using a second set of tyres, which was ultimately the best of the improvements.
“These practices are hard to read into exactly where you are,” Mostert said.
“We’ve got some work to do with the car, so we’ll just read on the balance.
“We’re struggling a little bit out there, but overall it wasn’t a bad hit-out first.
“I think the same guys will still be the guys to beat.
“Reynolds is always super-quick around here. We’ve got a bit of work to do.”
It dropped Reynolds to second, while van Gisbergen jumped up to third, having been down the order for much of the session.
“We rolled out pretty strong, made a little change for that second run, which was the best time that we did,” said Reynolds.
“Then we came in and made a couple more changes and tuned it backwards. We know what not to do now going forward.
“But it was a really good session to roll out and have strong pace straight away.
“I like driving this place. It’s got good flow, good rhythm. It’s fun.”
Lee Holdsworth, Scott Pye and Simona De Silvestro also improved late to fill fourth to sixth, with De Pasquale demoted to seventh.
Will Davison, Todd Hazelwood and Cameron Waters rounded out the top 10.
McLaughlin and Whincup were 11th and 12th, after earlier ends to their sessions, while Coulthard in 16th reported a minor throttle problem.
“We did a race run and I unfortunately had a lock-up into Turn 1 there,” McLaughlin said.
“It was pretty big and the flat spot was pretty big.
“We made a little change in the front that will probably affect my breaking.
“So we’ll go back there and find it a different way.”
Coulthard said his car felt "foreign" due to the throttle issue.
“It was just very light in the early phase and here with the temperature being so hot, you have to have that ultimate feel,” he said.
“I’m not making excuses; it was just an abnormality. Just felt a little bit foreign. The boys will tune it up, maybe put a new throttle spring on, something like that.”
Rick Kelly was the best of the Kelly Racing Nissans in 15th, but lost his final flyer to an off-track moment.
Deputising for the injured Richie Stanaway at Boost Mobile Racing, Chris Pither was 23rd-fastest in the 25-car field.
Practice 2 starts at 1:45pm local time on Friday.