The pilot was positioning for a spray run and the aircraft was in the last third of the procedure turn at an angle of bank of about 45 degrees. The aircraft was reported to have overshot the alignment of the next run slightly. One witness said he saw the nose of the aircraft go down. Another witness indicated that the aircraft then rolled under in the turn and struck the ground. The aircraft hit the ground inverted and travelled only about another 20 metres. An explosion occurred on impact and only the left wing and maingear remained unburnt. The pilot was wearing a full helmet and his harness was correctly worn and fastened. No apparent attempt to egress had been made. Subsequent investigation could find no faults or anomalies with the airworthiness of the aircraft that might have contributed to the accident. Advice provided to the investigation indicates that the Pawnee, at or near the stall in a turn, will stall the lower wing and roll under if aileron alone is used to try and pick up that wing. It was likely that while the pilot was manoeuvring to regain the alignment of the spray run, the aircraft was inadvertently stalled. A draft of this report was addressed to interested parties for consideration prior to release.