The Walkinshaw Andretti United pair were looking well in contention for a result at Mount Panorama, sitting fourth as the race neared its halfway point.
That was until Percat made a critical mistake on the run to Forrest's Elbow, a mistimed downshift sending his Commodore into the wall.
He was able to limp back to the pits for repairs and the car is now back in the race, however Percat and Luff are 10 laps off the lead.
"When I went to down change to second the dogs clashed in the gearbox and rejected the downshift," a dejected Percat told Motorsport.com.
"I ended up stuck between second and third and then in the fence.
"It's not ideal. The car was annoyingly good. I think it's one that got away. It was a comfortable podium car, to be honest, if not good enough for the race win."
Percat added that the mistake was particularly frustrating as he wasn't pushing or looking to overtake the third-placed #100 DJR Mustang, which was right in front of him at the time of the crash.
"I was fuel saving behind Anton, I wasn't risking anything," he said.
"That's the most annoying thing. I was just driving around.
"That's one thing around here, if you're not 10-10ths you can get lazy on things. It's my mistake. I didn't get down the gear properly."
