Tickford Racing's Chaz Mostert led the way in opening practice for the Beaurepaires Melbourne 400 in his Supercheap Auto Mustang.
Mostert turned the only sub-1:56s lap of the session, his 1:55.5743s finishing 0.4299 seconds clear of Anton De Pasquale with Jamie Whincup third.
Championship leader Scott McLaughlin was eighth while Shane van Gisbergen was down in 22nd.
Albert Park running got underway with the first of Thursday's pair of 30-minute practice sessions.
McLaughlin set the early pace with a 1:56.6542s, and spent the first half working through a long run in his Shell V-Power Racing Mustang.
That stood as the benchmark into the final four minutes, with McLaughlin two tenths clear of Whincup's Red Bull Holden.
An Albert Park winner in 2018, Scott Pye moved to the top briefly before Whincup went half-a-second clear on a 1:56.0824s.
Mostert made another big gain with to 1:55.5743s in the closing stages as De Pasquale improved to second, four tenths further back.
That was how it stayed at the front, with Mostert, De Pasquale and Whincup the top three.
“It was pretty tough for us, we had a lot of comms issues so I couldn’t really hear what Adam [De Borre, engineer] was saying and relay what I needed back from the car,” said Mostert.
“Putting a second tyre on there at the end really showed some pace in the car, but there’s probably still some tuning up to do.
“As a qualifying car it’s pretty good, which is nice to be in Practice 1, but not doubt there will be some cars coming for us in the next one.”
Of his session, De Pasquale added: “It’s good, we’re starting off each round we come to quite strong.
“It’s only Practice 1, a lot will change before qualifying later this arvo, but it’s good to start off strong.
“I ran a good set, I’m sure most of the guys in the [top] 10 did, bar a few, so it’s a good benchmark to start off.
“We’ve got a bit of work to do to make the car feel a bit nicer, but so far so good.”
Mark Winterbottom and Lee Holdsworth were fourth and fifth, from Mobil 1 MEGA Racing pair James Courtney and Pye.
McLaughlin did not improve on his early time, while Cameron Waters and Simona De Silvestro rounded out the top 10.
Will Davison only set one representative time after engine problems, at the very end of the session, but it was good enough to slot into 12th.
Nick Percat and van Gisbergen encountered splitter flap with their ZB Commodores and wound up 13th and 22nd.
While Percat's problem appeared isolated to the early stages, van Gisbergen did not set an indicative lap during the session.
“It was a bit of a shame, we had a bad splitter vibration,” he said.
“We pitted, tried about four times to fix it, there’s no point trying to push on and do more damage to the rest of the car.
“We’ll spend some time now to get it fixed, but it was a shame to lose the session.
“We’ll see what we can learn from [Whincup] and start the next one with their set-up I guess. It’s a shame, but it is what it is.”
Practice 2 of the Beaurepaires Melbourne 400 starts at 3:00pm on Thursday.