Mostert struck in the closing moments of the 30-minute session, jumping to third and then first on consecutive laps, recording a 1:11.0597s best.
That put him 0.0848 seconds clear of Caruso, who had knocked Shane van Gisbergen from the top moments earlier.
The order was frozen when red flags ended the session 67s early after Simona De Silvestro nosed her Nissan into the Turn 14 tyre wall and could not reverse out.
“I think that red flag at the end probably hurt a few people’s different tyre runs,” said Mostert.
“We did a bit of a different strategy, we ran our co-drivers at the start to get them comfortable.
“We know how much this race is as important for the co-drivers as the ‘mainies’. Hopefully that gives us a bit of an upper-hand on the other guys who are just putting their co-drivers out for the next one.
“The car felt OK, we’ve still got a lot of work to do. It’s not where we want it, definitely better than years past here, but we’ve got to get our head in the book and make it a bit better.”
Lee Holdsworth put his Preston Hire Holden fourth just prior to the red flag, while Scott McLaughlin was fifth after co-driver Alex Premat handled the final laps.
McLaughlin had struck the tyre wall on the exit of the opening chicane during his first run, scarring the doors of the Shell Ford.
It did not hurt its speed, however, with McLaughin leading the way for a period before van Gisbergen lowered the benchmark on three separate occasions.
“A bit of a balls up,” McLaughlin said of the brush with the wall.
“Saturday’s going to be wet, so we’re trying to find a bit of a confidence booster, that wasn’t it.
“[The rear of the car] was pretty nervous, we have to work on it.”
Bathurst winner David Reynolds ended up sixth ahead of Jamie Whincup, James Courtney, James Moffat and Mark Winterbottom.
“I scared myself a couple of times, which is always the objective in session one,” said Reynolds.
“It’s about finding the limit again, it’s been 12 months since you’ve been here, and our car is fundamentally different to last year.
“We’re a couple of tenths away from where we want to be, but we’ll find it.”
Despite a number of close calls as drivers felt out the challenging Gold Coast kerbs, the session ran without major incident.
Rick Kelly lost significant time in the garage while his Nissan crew attended to an early oil overflow that had also struck in opening practice at Bathurst, ending up 14th.
The second session, for co-drivers only, is scheduled for 1400 local time.