The pilot was taking his passenger for a flight to demonstrate the aircraft. Following preflight and pretakeoff checks the takeoff was commenced from runway 28. The aircraft accelerated normally until 60 knots when the aircraft was established in a 400 feet per minute climb. As the aircraft crossed the upwind threshold it began to sink. The pilot checked the performance and found that the airspeed was 60 knots and the engine was developing full power. He lowered the nose of the aircraft to the straight and level attitude and the aircraft maintained level flight. However, as the nose was raised again to outclimb trees ahead, the aircraft again began to sink. The pilot turned the aircraft to the right towards a clear area but was unable to avoid the trees. The aircraft came to rest upright in an area covered by 15 foot high regrowth. The pilot was able to extricate himself from the wreckage but because of his injuries could not free the passenger. The pilot had not operated from Mareeba prior to this flight and was unaware that local pilots do not normally use runway 28 for takeoff because of downdraughts near the upwind threshold. At the time of the accident the weather was fine and clear with a temperature of 22 degrees Celsius and no wind.