An emotionally disturbed young woman was allowed to board the aircraft as the sole passenger for the 25 minute flight. At top of climb, the passenger attacked the pilot and made an attempt to exit through the pilot's side window. In the process, she disturbed the aircraft and engine controls. The aircraft descended out of control from 4,500 ft and after pushing the passenger out of the cockpit, the pilot regained control by 3,000 ft. After being threatened by the pilot, the passenger remained subdued for some time. However, approaching Bundaberg, she again became agitated and harassed the pilot. The aircraft landed safely. Resort staff allowed the passenger to board the aircraft unescorted despite the availability of suitable staff. The passenger had been "playing up" all weekend had caused problems at the resort.