Defending Bathurst champions Garth Tander and Nick Percat are the first major casualties in Sunday's 1000km V8 Supercars endurance race at Mount Panorama.

Percat was behind the wheel of the pair's Holden Racing Team Commodore in the 38th of 161-laps when the vehicle suffered a mechanical failure and made contact with a wall at the top of the mountain.

The 24-year-old managed to limp the car back to the pits despite severe steering issues but Tander, who watched the drama from the team's garage, conceded their title defence was over.

"That's it, we're over and out for the day," the three-time Bathurst winner told the Seven Network.

"Try and fix her up and scoot around for the afternoon ... the young fella (Percat) looks pretty distraught but it didn't look like his fault."

Only one car has so far been forced to retire from the race after 40 laps, with Michael Patrizi and Jonny Reid pulling out after their Commodore suffered a driveshaft failure.