A Beech B200 Super King Air aircraft, registered VH-SGT, sustained damage when its main landing gear collapsed on touch down on runway 24 at Perth Airport, Australia. The aircraft had earlier departed the aerodrome on a charter flight with one pilot and seven passengers on board. The pilot had elected to return to the airport after detecting a problem during the retraction of the aircraft’s landing gear. He attempted to manually extend the landing gear, but was unable to confirm that the landing gear had locked in position for the subsequent precautionary landing. The main landing gear collapsed as the aircraft touched down. Examination of the aircraft landing gear by the aircraft operator revealed an incorrectly installed thrust bearing in the hydraulic actuator for the left main landing gear. The result was that the axial loads normally carried by the thrust bearing were transferred to the actuator’s pinion gears which fractured under abnormal operational loads. That damage seized the actuator and prevented the manual extension of the aircraft’s landing gear by the pilot.