The first of two 15-minute sessions on Saturday morning was topped by Purdie in the #48 Audi R8 with a 1:09.8210s lap.
Kiwi young gun William Exton (#114 McLaren Artura) slotted into second, just 0.14s behind Purdie, with championship leader Tom Hayman (#25 McLaren) in third.
Blake Tracey (#35 Ford Mustang) continued the strong form shown in practice running fourth fastest, while lead Silver-Am competitor Zac Soutar was the third McLaren entry in the top five.
A pair of Ford Racing junior drivers were sixth and seventh, with American Sam Paley (#62 Mustang) closely followed by Cameron McLeod (#227 Mustang).
Shearer (#46 Porsche Cayman) was eighth, followed by Blake Dowdall (#95 Mustang) in the second Miedecke Motorsport entry.
Aaron Seton (#1 Mustang) rounded out the top 10 in the repaired Gomersall Motorsport entry.
Click here to find out how.
Nathan Callaghan led the way in the Am Cup in 19th outright, just 0.04s clear of class championship rival Glenn Walker.
Qualifying 1 ran unaffected for the entire duration, with no drivers finding trouble.
Two-driver entries swapped pilots for Qualifying 2, where Shearer set the pace with a 1:09.8598s to seal his first GT4 Australia pole position.
Shearer led home Jett Murray (#25 McLaren) by the narrowest of margins, with just 0.08s covering the pair.
Jobe Stewart rounded out the top three in the #48 Audi.
Glenn Nirwan was the fastest Silver-Am competitor in 14th outright, with the #75 McLaren sweeping both sessions in class.
Jarrod Keyte (#21 Mustang) took the honours in the Am Cup in 15th outright.
Several cars found trouble in the second session, including third-placed Stewart.
The Supercars rookie ran wide on the exit of Turn 1, taking light damage the the left-rear bodywork on the Audi.
Jack Johnson went sixth fastest despite a spin on the exit of Turn 1, while Nirwan found the grass at Turn 5.
Super sub Sam Brabham was 12th fastest in his cameo return to the category.
Monochrome GT4 Australia returns for the first of two one-hour races this weekend at 1:10pm ACST, live on Fox Sports and Kayo.
Discussion about this post