On arrival in the circuit area at Cooinda the pilot selected the landing gear down but no 'down' indication was obtained for the left maingear. After several unsuccesful attempts to obtain a left gear down indication, the pilot decided to return to Jabiru where company ground staff were available. He contacted Adelaide Flight Service with a request to telephone his company, but did not advise of any problems with the aircraft. At Jabiru several normal and emergency extensions of the landing gear were carried out, each time with the same unsafe indication. Several passes were flown to enable ground staff to confirm the position of the landing gear, which appeared to be down. A normal approach and landing was made but as the aircraft touched down the left landing gear leg collapsed. The pilot was able to hold the wing up for a short time, but as the aircraft slowed the left wing settled onto the runway, turning the aircraft through 180 degrees. Because the pilot had not advised Adelaide Flight Service of the problem the aerodrome emergency procedures were not activated. Subsequent investigation revealed that the left hand extension/retraction mechanism torque tube rod attachment had failed due to fatigue.