The Mooney was landing at Mackay after an unevenful flight from Brampton Island. During the landing roll the right main landing gear slowly retracted until the wing was scraping on the ground. The gear extension system for this model aircraft is manually operated by the use of a handle in the cockpit. The pilot advised that the handle was down and locked prior to landing. A subsequent investigation found that actuation of the landing gear was inhibited due to rust on the surface of the metal drive rods and plates. As a result the right main landing gear had failed to lock into place even when the extension handle was locked down. The right main landing gear slowly retracted as the aircraft settled onto the partially extended leg. The operator's maintenance facility advised that the aircraft had not flown for an extended period, and that the area where the aircraft was normally parked is close to the sea.