The undercarriage of a Beechcraft B200 Super King Air (registered VH-SGT) collapsed on touchdown at Perth airport. The aircraft was extensively damaged as a result of the collapse. No injuries were sustained by the pilot or passengers from the accident. Approximately two hours earlier, the aircraft was chartered to fly from Perth to Mount Hale, WA when shortly after takeoff from Perth the aircraft experienced a malfunction of the landing gear system. The main wheels and nose gear had become jammed and were unable to fully retract when selected up by the pilot. The pilot completed the emergency checklist actions contained in the Aircraft Flight Manual, but was unable to retract or extend the gear using either the automated control or the manual emergency system. The pilot then requested assistance from a passenger to operate the manual emergency extension system. The landing gear remained jammed despite the additional force applied to the lever from the passenger. The pilot contacted air traffic services and requested further assistance from company engineering personnel to visually assess the extension state of the landing gear. Two aerodrome passes were completed throughout the troubleshooting exercise and the pilot remained in radio contact with both groups during this phase. Following the flyovers and after holding over Rottnest Island at 5,000 ft for a period of approximately two hours, the pilot flew the King Air back to Perth airport. With the gear still jammed in the partially retracted position, both the left and right main landing gear assemblies collapsed after the aircraft touched down on runway 24. The aircraft was substantially damaged as a result of the collapse. The Australian Transport Safety Bureau investigation revealed that two major system components had failed which could have prevented the landing gear from properly retracting/extending; the geared components within the right main landing gear actuator had fractured, and the left torque tube support bearing had seized from contamination and lack of lubrication. Although each component failure was apparently unrelated, the examination was not able to conclusively establish which failure had been the primary contributing factor in this landing occurrence. The Super King Air 200 aircraft landing gear system configuration was such that should either one of these component assemblies cease to function, extension or retraction of the landing gear would not have been possible.