The aircraft was making a landing approach to runway 03. The pilot reported that the left main gear was indicating an unsafe condition and carried out an overshoot. System checks were carried out without success. An attempt was made to extend the landing gear with the emergency extension system but the left main gear continued to indicate an unsafe condition. Two fly-pasts were carried out and the Tower reported the landing gear appeared to be extended normally. With the airport emergency services standing by the aircraft made a normal approach. Shortly after touchdown the left gear collapsed resulting in the aircraft slewing left and departing the runway. The pilot reported that a subsequent examination revealed low hydraulic system pressure may have been a factor in the failure of the landing gear to extend normally.