Commodore rolling backwards

IF YOU check your owner's manual you will see Holden recommends using the brake to hold your vehicle on hills. That said, it is unusual to have to use the brakes to stop the car rolling backwards. It's possible the idle speed is on the low side and the torque converter is also low in terms of take-up. If you're not happy with Holden's response, argue your case again. On the face of it I think you have one. You could also consult an auto trans expert to make sure all is well.