He was more than half a second up on the field for the majority of the session but former team-mate Nick Percat eventually closed the gap to 0.1753s by the end of proceedings.
Whether anyone would have got closer had an incident for Cameron Hill not brought an early end to the session is not known, although Mostert’s focus was not on Hollywood lap times by then.
“Obviously, I have no idea really what happened in the session but, for us, we kind of just stuck to our plan,” he said.
“Car felt pretty good out the gate but we went pretty aggressive in the session and probably made it a lot better.
“We, focused more on our race car towards the end of that practice, and I think our race car was actually quite good.
“We’ll see how we go tomorrow; tomorrow’s 20-minute session, for us, was always about focusing going into qual[ifying].
“So, pretty happy with how it kind of went but you know what it’s like – Friday practice with the new rules [lower minimum tyre pressure] and stuff like that – who knows what tyres people are using?
“But, at the moment, I’m pretty happy with our race car.
“No doubt there’ll be some fast cars out there tomorrow as well.”
Mostert went on a relatively long final run, consistently producing mid- to high-0:55s.
It comes after a relatively early mid-season test for WAU, which visited Winton earlier in the month, and early indications are positive.
“We’ve kind of implemented a lot coming in this weekend,” explained Mostert.
“It was a bit of an up-and-down test day. Probably ‘til lunchtime at the test day, I was really, really happy, and then we kind of didn’t get much of a read in the afternoon.
“This session today was trying to test those further items that we wanted to do that we didn’t quite get a read on and I still thought they were probably quite good.
“Touch wood, I think we’ve got a better platform than what we had for the first three rounds of the year.
“Here is a very unique track, too. The track was super-hot, it doesn’t give you a lot of feedback and grip, but our car worked a lot better than what it does even on a high-grip track.
“So, we’ll take today, we’ll go through some data, we’ll see if we can make it faster tomorrow.
“I had little Everly [newborn daughter], before I came, pick some fast socks for me for the weekend, so hopefully I haven’t peaked too soon on the favourite pair of socks; I might have to commit to the next three days with that pair.
“But, it’s awesome, it was really nice to drive, so I’m looking forward to the weekend.”
Earlier, WAU also set the pace in Super2 practice, courtesy of Zach Bates.
Practice 2 for the Supercars Championship field, a 20-minute session, starts tomorrow at 09:15 local time/11:15 AEST.