The flight consisted of one aircraft in a group of three positioning a mobile polling booth. After the passengers were unloaded at Oenpelli, the aircraft flew to Jabiru to refuel and the pilot rested in the air conditioned snack bar for about five hours. The aircraft was then re-positioned at Oenpelli and the passengers boarded. The aircraft departed from runway 12 as the wind was 120 degrees at 20 knots, gusting to 25 knots. The temperature was about 40 degrees Celsius. This was not the pilot's preferred runway as the Spencer Range escarpment commences within five miles of the DEPARTURE end of this runway, rising rapidly from sea level, through 500 feet to over 1,000 feet. At some point after take-off, the pilot realised the left engine manifold pressure was steadily decreasing. After completing trouble checks, he feathered the left propeller and turned back towards the aerodrome. When turning onto the base leg, the pilot realised the aircraft would not maintain altitude and could not reach the runway. He then elected to land straight ahead on a dry clay pan. The landing gear could not be extended as the left engine drives the hydraulic pump and insufficient time remained for the pilot to manually extend the gear. The aircraft touched down wings level and skidded for about 135 metres. All four occupants evacuated safely once the aircraft stopped. The investigation failed to determine any pre-existing defect in the left engine. However, impact damage to the undersurface of the engine, may have destroyed any evidence of a pre-existing defect.