The Tickford Racing driver started Friday evening's 25-lap encounter from 22nd after two mistakes in qualifying saw him buried for the opening race of the weekend.
“If we can unlock the same qualifying speed we might be someone to beat out there.
“Credit to Adam (De Borre, Mostert's engineer); through last year we really struggled with race pace and now we've turned the thing into a bit of a monster in race conditions.
“Wish we could run full tanks all the time, car felt pretty good. I'm shocked to get the fifth to be honest, it's a real credit to Adam to make the tyres live.”
Mostert admitted tagging one of tyre bundles on the apex of some corners, which caused his front right wheel to briefly bellow smoke.
“First couple of laps with 21 cars in front of you, you kind of misplace tyre bundles,” he said.
“Also when someone else hits it, it also bounces it out a bit further into your car.
“Clobbered it twice pretty bad, but credit to our crew on 55. They've put some strong bolts in the front so it held together pretty good.”
Mostert qualified second for Race 4 of the championship which gets underway on Saturday at 1250 AEDT.