A CH-75E Caterpillar Challenger Tractor towing a pipe line tamer trailer was stringing 110mm HDPE water pipe in a gas gathering area. When reversing the line tamer into position on an incline to commence unloading two coils of pipe, the Tractor engine stalled and the machine rolled forward down the incline for more than 250m. It narrowly missed a 36 tonne Hitachi excavator and came to rest in a creek bed. Nobody was injured and equipment damage was minor.