The in-form Erebus squad set the early pace, Will Brown leading Kostecki with a 51.571s after their first runs.
At the seven-minute mark they were both jumped by Le Brocq's 51.404s.
Broc Feeney made big gains five minutes later as he popped up on top courtesy of a 51.331s. It was a short-lived stint as the pacesetter, though, with Le Brocq returning serve with a 51.243s moments later.
Not long after there was drama for Anton De Pasquale, his Shell Mustang losing drive on the back straight.
He tried to roll back to the pits but only made it to pit entry. The crew pushed the car back to the garage but he was out for the remainder of the session.
At the same time there was contact at turn 2, Andre Heimgartner helping rookie Declan Fraser off the road.
With a couple of minutes to go a number of drivers switched to green tyres, Cam Waters looking to threaten Le Brocq's time until he hit traffic in the last sector.
The times did eventually tumble, Kostecki ending up on top thanks to a 51.172s right at the flag.
Le Brocq improved to end up second with a 51.191s while Brown slotted into third.
Thomas Randle led the way for Tickford, and was one of just two Fords in the top 10, in fourth ahead of Mark Winterbottom, Shane van Gisbergen and James Golding.
Broc Feeney was eighth after having his final run delayed when his Red Bull Camaro momentarily wouldn't fire up, with Tim Slade ninth and Todd Hazelwood 10th.
James Courtney, meanwhile, survived a scary moment late in the session when he arrived at full speed to fine a train of slow cars at turn 6. Most of the cars stayed off the racing line, except David Reynolds who strayed across the track while warming his tyres.
Courtney was forced to take to the grass at speed to avoid his Penrite Mustang, the veteran lucky to avoid any serious damaged.
The pair were shown on the broadcast speaking about the incident post-session with Reynolds apologising for not seeing Courtney coming.
"I was flat out and everyone was going over to the left," said Courtney. "We have this little drivers' group and everyone said yes, if you're not on a lap, you'll go on the left.
"I thought I'd keep it pinned and then Davey weaved out and I had to get out of the way. It got pretty exciting.
"He apologised, said he didn't see me. All the normal stuff. It's just characteristic of this track, we all know it's going to happen."
Reynolds added: "We have our lights on to tell people we're on a hot lap, but the Camaros have a daytime running light on all the time, so you can't really tell who's on a hot lap and who's not.
"I looked miles back and I couldn't see anyone coming and started weaving and I looked right and there was Courtney's car and I shit myself. I just went down and apologised, I didn't mean it."
The next session for Supercars is qualifying at 12:55pm local time.
