On approach into Melbourne the landing gear was selected to the down position for landing. The usual thump of the nose gear uplock unlatching and subsequent air noise was not evident. The nose landing gear had not unlocked and only two green indicator lights illuminated for the main gears. The aircraft was vectored away from the circuit where a manual gear extension procedure succeeded in lowering the nose gear. The aircraft returned for a normal landing. Investigation disclosed that a bush on the nose landing gear drag brace had seized. The aircraft's normal landing gear extension system was unable to overcome the drag created by the siezed bush. It is suspected that inadequate lubrication or excessive washing out of the lubricant was responsible for the bush seizing. A fleet check was carried out, however, all other bushes were found to be satisfactory.