Mostert finished third in the opening two sprints.
“One day, you’re right smack bang in the middle and you’re happy as, and one day you’re just a little bit out of it,” he said.
“Not really sure what to think about that.”
Local product Brodie Kostecki’s hopes for a home race victory were dashed when a poor qualifying effort condemned him to 15th.
Kostecki finished the two opening sprints in Perth in fifth and seventh.
Drivers returned for the top-10 shootout at 1.05pm (AEST), before the final sprint in Perth at 3.15pm.
Race 16 qualifying results:
1. Broc Feeney (Triple Eight Race Engineering)
2. David Reynolds (Team 18)
3. Matt Payne (Grove Racing)
4. Will Brown (Triple Eight)
5. Bryce Fullwood (Brad Jones Racing)
6. Ryan Wood (Walkinshaw Andretti United)
7. Cam Waters (Tickford Racing)
8. Cam Hill (Matt Stone Racing)
9. James Golding (PremiAir Nulon Racing)
10. Anton De Pasquale (BJR)
AAP
