The aircraft was engaged on a training flight involving instrument flying and circuit practice. At Griffith the pilot carried out a VOR let down from which the aircraft was positioned for a circling approach to runway 24. The landing gear was extended during the instrument letdown procedure and the cockpit indicator showed the landing gear was safely down and locked. When the aircraft was on final approach, both pilots checked that the landing gear indication was unchanged. The pilot reported a normal touched down, when without warning, the right maingear leg collapsed. The pilot was able to bring the aircraft to a halt on the runway. The wind conditions at the time were variable at 5 knots. A technical examination revealed the right main landing gear had collapsed as a result of material failure. It was determined that the lock nut which secured the rod end of the piston rod of right hydraulic actuating cylinder had loosened. This permitted the rod end to screw into the piston rod, shortening the length of the piston rod. The shortening of the hydraulic actuating cylinder rod travel prevented the drag brace from being positioned in the over centre position to complete the extension cycle. The locknut had progressively loosened due to contact with the rear end of the clevis yoke in the final stages of leg extension. As a result, the loads from the right maingear were taken by the right hydraulic actuating cylinder and its mountings. The right clevis on the landing gear strut consequently failed in overload. The pilots were unaware of an unsafe right maingear. The down lock micro switch was incorrectly adjusted, such that a green light "safe" indication appeared in the cockpit before the drag brace was in the over centre or locked position.