A troublesome crank-angle sensor meant his Wilson Security Commodore nearly missed the start, then stopped on lap four while running seventh, losing six laps in pitlane.
From 18th on the grid on Sunday, Tander shot into 11th on the opening lap but copped a massive hit from Andre Heimgartner's Nissan entering pitlane on lap three, sustaining heavy damage to the rear of his ZB.
"A weekend with a lot of promise but unfortunately unfulfilled," Tander surmised.
"[A] drama with a misfire on Saturday; running in the top 10, the car felt really strong and had that misfire that cost us a lot.
"Then Sunday, got caught out in qualifying with all of the dust, got a good start and first lap, made up a heap of spots.
"I thought our strategy was going to be right and as I came into pitlane for the first pitstop I got absolutely drilled by Andre Heimgartner, completely wrecked the back of the car.
"Frustrating, really disappointing for the guys, because I thought we had a fast race car, but it wasn't to be."
The contact followed a clash between Tander and Heimgartner at Sydney Motorsport Park earlier this month, with Tander musing he might "paint [the car] fluoro orange for the next race so he can see us!"
Heimgartner, who retired from the race due to the front-end damage, said: "I was just behind a tight group and we all entered pitlane at the same time.
"I misjudged the distance to the car in-front and made contact. It was my mistake and I’m sorry to Garth for the incident."
Rookie team-mate James Golding started the races 16th and 25th and finished 21st and 24th, lamenting early damage each time.
Stewards investigated an incident involving Golding and wildcard Kurt Kostecki on Sunday's opening lap but took no further action.
"Not the greatest weekend for us, results-wise," said Golding.
"A bit unfortunate to get damage at the start of each race, which bent our front bumper and lost a lot of aero, which around this track's really important.
"There's a couple of glimpses here and there we can take away as positives, but other than that we've got to put our heads down and do a better job at the enduros."
Tander and Golding start the PIRTEK Enduro Cup at Sandown next month 14th and 23rd in the Supercars standings.