The pilot had flown from Adelaide to Bacchus Marsh, on 25 July 1980, and had left the aircraft there while he attended to private matters. On his return during the afternoon of 1 August he discovered that there was no fuel available, and elected to fly to Essendon to refuel and to obtain a pre-flight briefing. It had been the pilot's intention to proceed as far as Horsham and remain overnight before continuing, however the weather forecast was satisfactory and he decided to plan a Night VMC flight to Adelaide. The pilot noted on the plan that the lowest safe altitude on the initial leg to Yarrowee was 3500 feet above mean sea level (amsl), and was 4800 feet amsl between Yarrowee and Mt William. Planned cruising altitudes were below 4000 feet amsl to Yarrowee, thence below 5000 feet amsl. About 25 minutes after DEPARTURE the pilot was advised that the weather at Adelaide was deteriorating and he elected to divert and land at Horsham. Amended flight details were passed, indicating that the aircraft would proceed to Horsham from over Mt William, cruising below 5000 feet amsl. At 2049 Melbourne Flight Service contacted the pilot and requested an estimated time of arrival for the Mt William position. The pilot replied that his revised time for the position was 2051. This was the last transmission received from the aircraft. At about 2050 witnesses noted an intense fire near the summit of Mt William. The following day it was established that the aircraft had collided with the almost vertical face of the mountain, some 400 feet below the summit, while apparently in cruising flight at about 3400 feet amsl and on the track from Yarrowee. The wreckage had then fallen about 200 feet to sloping ground and had been consumed by an intense post-impact fire. The weather at the time of the accident was reported as fine, with good visibility. Although the winds were stronger than forecast, it was considered that there was no significant turbulence or downdrafts in the area. The examination of the wreckage was hampered by the extent of the damage, however no evidence was found to indicate any pre-impact defect which might have contributed to the development of the accident. The reason the pilot was flying the aircraft more than 1000 feet below the lowest safe altitude for the route sector could not be determined.