Mostert scored pole for Race 3 in the knockout session, pipping Cameron Waters to P1 with a 1:07.630s.
The 28-year-old’s Q2 time was also the fifth quickest of the three-part session.
Mostert also scored the quickest race lap of the weekend, firing in a 1:08.554s on lap 10 after taking on new tyres in Race 3.
He eventually finished sixth, with his rivals racing past him on fresher rubber late in the race, Shane van Gisbergen completing a remarkable 17th-to-first victory.
Van Gisbergen set the third quickest race lap of the weekend in his stunning charge.
Sandwiched between Mostert and van Gisbergen was Jamie Whincup, who regained form after a lowly Bathurst showing.
Whincup’s run to third in Race 4 on Sunday, which began in the wet, featured a 1:08.876s flyer on lap 28.
Race 5, held in entirely wet conditions, saw van Gisbergen log a 1:18.607s best to score the additional five championship points.
The Repco Supercars Championship field will return to Symmons Plains for the Beaurepaires Tasmania SuperSprint across April 10-11. Tickets are on sale now.
The event will be broadcast live on Foxtel and will be streamed on Kayo.
Fastest practice laps
Session
Driver
Time
P2
Cameron Waters
1:07.912
P2
Mark Winterbottom
1:07.917
P1
Jamie Whincup
1:08.070
P2
Anton De Pasquale
1:08.139
P2
Chaz Mostert
1:08.147
P1
Cameron Waters
1:08.159
P2
Shane van Gisbergen
1:08.182
P2
James Courtney
1:08.182
P2
Jack Le Brocq
1:08.216
P2
Scott Pye
1:08.244
Fastest qualifying laps
Session
Driver
Time
R3/Q3
Chaz Mostert
1:07.630
R3/Q3
Cameron Waters
1:07.675
R3/Q3
Jamie Whincup
1:07.788
R3/Q3
Mark Winterbottom
1:07.855
R3/Q2
Chaz Mostert
1:07.881
R3/Q2
Mark Winterbottom
1:07.944
R3/Q3
Jack Le Brocq
1:07.964
R3/Q2
Jamie Whincup
1:08.033
R3/Q3
Anton De Pasquale
1:08.036
R3/Q2
Jack Le Brocq
1:08.118
Fastest race laps
Session
Driver
Time
R3
Chaz Mostert
1:08.554 (lap 10)
R4
Jamie Whincup
1:08.876 (lap 28)
R3
Shane van Gisbergen
1:08.913 (lap 27)
R3
Zane Goddard
1:08.965 (lap 10)
R3
Will Brown
1:08.972 (lap 11)
R3
Cameron Waters
1:08.972 (lap 7)
R3
Anton De Pasquale
1:08.889 (lap 14)
R3
Jamie Whincup
1:09.002 (lap 10)
R3
Mark Winterbottom
1:09.008 (lap 10)
R3
Tim Slade
1:09.084 (lap 25)
Key: Practice (P); Qualifying (Q); Top Ten Shootout (TT); Race (R)