The aircraft was engaged in a passenger charter operation from Moree to Tenterfield. After landing at Tenterfield, when the aircraft turned off the strip, the left main landing gear broke away and the aircraft settled on the left wing. Prior to the previous takeoff, whilst the aircraft was taxiing at Moree, the landing gear aural warning had sounded momentarily. The pilot and an engineer inspected the landing gear down lock system but no fault was found. The aircraft then operated normally to Tenterfield until the aircraft turned off the strip. Subsequent investigation of the landing gear showed that the forward trunnion pin had became disengaged from the trunnion and the rear pin had migrated some distance aft and finally failed in overload. Movement of the trunnion pins was the direct result of incorrect assembly procedure.