Kostecki continued Erebus's dominance at Mount Panorama by clocking a 2:04.6644s in the #99 Coca-Cola Camaro, with PremiAir Racing's James Golding set to be second-last out in the Top 10 Shootout after finishing 0.1489s off the pace in the #31 Nulon Camaro.
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Broc Feeney took fifth in the #88 Camaro while his Red Bull Ampol Racing team-mate Shane van Gisbergen rescued a shootout berth when he vaulted six positions to seventh on the timesheet with the chequered flag out.
It was a crash-marred opening to what was supposed to have been a 40-minute session.
No one had set a lap time when Brown locked up and ran into the barriers at Griffins Bend, bringing about a red flag in the fifth minute of proceedings.
The #9 Coca-Cola Camaro was pulled out of the tyre belt and Brown drove back around the track, during which time he reported that steering was straight, and the Erebus crew went to work on repairs in their pit bay when he returned to the lane.
After a stoppage of nine minutes, the session went green again, but not for long enough for anyone to complete a flying lap.
This time it was Declan Fraser coming to grief, and in a more significant fashion, when he smashed into the concrete wall on the climb to Reid Park and caused session-ending damage to the #56 Tickford Mustang.
Officially, everyone except Brown had a time on the board by then, but it was Cameron Hill (#35 Truck Assist Camaro) who was on top with a 10:24s on his previous out lap.
By the time Car #56 was recovered, the extra 15 minutes had been exhausted and the session resumed with 30:02s left on the clock.
Triple Eight Race Engineering's Feeney was first to set a flyer, a 2:05.3575s, but Waters (#6 Mustang) was first ever into the ‘fours' in the Gen3 era with a 2:04.9909s.
Kostecki was threatening to go quickest when each of the two red flags were called, and finally did so on the second lap of this third run with a 2:04.6644s, despite tailing Mark Winterbottom through Forrest's Elbow.
Meanwhile, Erebus had fixed team-mate Brown's #9 Camaro and he would clock a 2:05.8700s to go 15th with his first flyer of the session.
With 20 minutes remaining, Brodie Kostecki held sway from Waters, Golding, and Matt Payne (#19 Penrite Mustang), while van Gisbergen (#97 Camaro) was 26th without a representative time after running long at Hell Corner and Scott Pye (#20 Toyota Forklifts Camaro) was back in the pits after bumping the wall at Griffins Bend.
Golding then moved to second on a 2:04.8133s and De Pasquale (#11 Mustang) to third on a 2:04.9667s, before van Gisbergen finally strung together a 2:05.4739s to move into 10th.
Team 18 looked over the Pye Camaro and it was reportedly found he had only knocked the rear toe, so the two-time Bathurst 1000 podium finisher was sent again and took up 15th on a 2:05.6401s.
The final runs started with six minutes to go, at which point Kostecki was still on top from Golding, De Pasquale, Waters, Feeney, Payne, Will Davison (#17 Shell V-Power Mustang), David Reynolds (#26 Penrite Mustang), Thomas Randle (#55 Tickford Mustang), and Chaz Mostert (#25 Mobil 1 Optus Mustang) in 10th on a 2:05.4118s.
Notables definitely needing to improve to make the Top 10 Shootout were van Gisbergen in 11th, Brown in 14th on a 2:05.5581s, and Andre Heimgartner (#8 R&J Batteries Camaro) in 15th on a 2:05.5694s.
Brown bailed on his next flying lap and in fact pitted before the session had finished, as did Kostecki.
Van Gisbergen also gave up the first flyer of his final run but pressed on for another.
Randle was up to eighth with a 2:05.3799s but was still vulnerable, especially when Reynolds moved up to sixth on a 2:05.0830s and then Mostert to eighth on a 2:05.2177s despite kicking up the dirt at McPhillamy Park.
That left Randle 10th when the chequered flag came out, after which van Gisbergen clocked a 2:05.1823s which had the effect of knocking the #55 Mustang out of the shootout.
Provisionally, the top 10 on the starting grid is Kostecki, Golding, De Pasquale, Waters, Feeney, Reynolds, van Gisbergen, Payne, Mostert, and Will Davison.
Randle is locked into 11th, from Pye, Heimgartner, Hill, Jack Le Brocq (#34 Truck Assist Camaro), James Courtney (#5 Tickford Mustang), Brown, Tim Slade (#23 Movember Camaro), Winterbottom (#18 DeWalt Camaro), and Zane Goddard (#888 Supercheap Auto Camaro) rounding out the top 20.
Tomorrow's Supercars Championship action starts with Practice 5 at 10:00, followed by Practice 6 at 13:00 and the Top 10 Shootout at 17:05 (all times local/AEDT).
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