During an early morning inspection of Broken Hill aerodrome the groundsman discovered an aircraft upside down about 300 m short of runway 23, just outside the aerodrome boundary. The owner was contacted by the local police who advised them that his aircraft had been stolen. Further police enquiries revealed that the owner had actually been the pilot, who now claimed he was suffering from amnesia, possibly caused by a head injury as the aircraft rolled over. Later the pilot stated that he had departed his property at approximately 2000 CSuT for Broken Hill, with an arrival time of 2016 CSuT. He joined the circuit on downwind for runway 23, reduced engine power and lowered 20 degrees of flap. Turning base leg he recalled that the engine appeared to be sluggish to throttle movement when an increase in power was required to overcome an excessive sink rate, which he thought may have been caused by the very high temperature of the day. He then retracted the flaps as he was worried that the aircraft might stall. This action probably increased the sink rate. The aircraft contacted the ground heavily well short of the runway, continued for several metres before contacting obstacles and rolling inverted. A friend and his family, who had visited the pilot's property that day for a barbeque, reported they had observed the pilot consume about 8 - 10 stubbies of full strength beer during the afternoon. They departed for their property at about 2000 CSuT, and after about 15 minutes were "buzzed" by an aircraft they believed belonged to the pilot, which then continued in the direction of Broken Hill. A thorough inspection, and test run of the engine was carried out which did not reveal any faults or malfunctions that may have caused a loss of power. An inspection of the aircraft failed to find any faults which could have contributed to the accident. The police and the Civil Aviation Safety Authority are investigating further.