Craig Lowndes led a Red Bull Holden Racing Team one-two in a rain-hit Winton practice session for additional drivers.
The session was the year's second aimed at co-drivers ahead of the PIRTEK Enduro Cup, which starts at Bathurst in October.
Rain largely nullified the value of the 30 minutes, though, ahead of opening Truck Assist Winton SuperSprint practice for full-timers.
Lowndes set a 1:24.1662s early in the greasy conditions, to sit 0.3889 seconds clear of team-mate Garth Tander.
The veterans are joining Jamie Whincup and Shane van Gisbergen in the enduros.
Times were more than five seconds off the lap record, though, and light rain started to settle in.
Roughly a third of the field parked up, while the balance opted to test the conditions on wet tyres.
The order was unchanged, Lowndes and Tander followed by third-placed Warren Luff, 1.1161s adrift.
"It’s a shame," said Tander of the conditions.
"With the format of this weekend with only one practice session for the primary drivers, every co-driver up and down pitlane probably had a workload to do this morning in the dry.
"One of those sessions where it’s not wet or dry, you don’t really learn anything, so that’s why we pitted, two laps, pitted...
"The wet tyre allocation is pretty tight as well, so there’s no point wasting wet tyres on us."
Tickford Racing's James Moffat and Luff's Mobil 1 MEGA Racing team-mate Jack Perkins rounded out the top five.
Perkins' session included an off at the Turn 5 sweeper, from which he recovered unharmed.
"I didn’t have any moments, I just wanted to get back to the pits quicker, it’s a shortcut through there!" Perkins said.
"It was nothing too serious, it’s practice.
"Come Bathurst podium, nobody will remember the Winton practice session, I can guarantee you."
Penrite Racing gave 16-year-old Kumho Super3 Series driver Broc Feeney a run, and he wound up sixth.
Dean Canto was seventh in Macauley Jones' CoolDrive Commodore, while Thomas Randle, Bryce Fullwood and wildcard Jack Smith rounded out the top 10.
Randle is expected to contest the Enduro Cup with Lee Holdsworth, while Fullwood's role alongside Andre Heimgartner was announced earlier this week.
Alex Premat flew to Australia from Las Vegas, only for rain to curtail his first taste of Shell V-Power Racing's Mustang.
He was ultimately 21st fastest of the 25 drivers, with a time set on wet tyres in the closing stages.
“Any time in the car is important,” he said.
“Driving the car, gaining the process with the team, how to talk with the guys, and even the feedback.
“They didn’t run so much on the damp condition with this new car, so it’s nice to get as much mileage as we could.”
Practice 1 for the full-timers starts at 10:40am local time.