The DC9 freighter crew performed a missed approach to Melbourne runway 27 when they did not get a landing gear safe indication after gear extension. The crew carried out a manual extension of the gear and obtained the safe indication, then proceeded to a safe landing. Because the landing gear doors remain open after manual extension the crew stopped the aircraft on the runway to allow engineers to secure the doors. This was the second of three similar events (refer BASI occurrences 9805884 and 9805887). After two failed attempts to re-rig the landing gear system it was found that normal extension could only be achieved if the system was rigged outside of maintenance manual tolerances. The mis-rig problem has been referred to the aircraft manufacturer and the aircraft was cleared for flight. The aircraft has subsequently operated satisfactorily