Anton De Pasquale has been excluded from the Race 21 results, and the Shell V-Power Racing Team fined, over a tyre mix-up.
De Pasquale started from pole position for the race, and crossed the line fifth.
It was later confirmed that a tyre from teammate Will Davison's allocation had been placed on De Pasquale's car during his pit stop.
The Saturday night race winner, De Pasquale made a poor start to the race and dropped to seventh.
De Pasquale was called in at the end of lap 8, and he took on new right-hand side, working side tyres.
Click here for updated Race 21 results
The undercut would return De Pasquale to the effective lead, but he was demoted to fifth by race's end.
Eventual winner Shane van Gisbergen and teammate Davison both went past on lap 23, with Nick Percat and Brodie Kostecki also going past in the closing laps.
De Pasquale remains sixth in the standings, but heads into Race 22 with a reduced 52-point margin over Percat.
The stewards' report reads: "Following the Race, the DRD received a report from Supercars Technical that during Race 21 the Authorised Representative of Racing Team (Aust) Pty Ltd had reported to the HoM that during Race 21 the Team had fitted a Control Tyre allocated to Car #17, Will Davison, on Car #11 in breach of Rule D17.1.6 which prohibits the transfer of a Control Tyre between Cars except as permitted by the Rules.
"The Control Tyre transfer between Car #17 and Car #11 was not permitted.
"The Team having admitted to the Breach, the Stewards imposed a Penalty of Disqualification of Car #11 from Race 21 and a Fine on Racing Team (Aust) Pty Ltd in the sum of $20,000 of which $10,000 is suspended until 31 December 2021 subject to the Team not committing any further breach of Rule D17 before that date."
Team principal Ben Croke confirmed the "mishap" happened in the pit stop.
"The wrong tyre was fitted to Anton’s car. It’s a mistake that we made, we’ve put our hand up, we accept the consequences," Croke said.
"It’s been a really positive return to racing for us, we have had really good cars here this weekend.
"So for us now, the focus is just to go out in this next one and do the best job we can.
"Anton is on pole [for Race 22], so we will move on and keep pressing on."
De Pasquale's exclusion elevates Tim Slade to fifth, with Jamie Whincup, Chaz Mostert, Andre Heimgartner, David Reynolds and Todd Hazelwood rounding out the top 10.
Race 22 will commence at 4:25pm AEDT.