VH-RMG was cleared to land on runway 19. VH-HSS was cleared to line up after VH-RMG had landed and told to expect an immediate takeoff clearance. At about this time VH-TJQ was approaching final and about 12 kilometres from the runway. At the end of its landing roll VH-RMG turned into a highspeed exit and stopped with the rear of the aircraft over the runway and with the entire aircraft infringing the flight strip. The radio transmitter of VH-RMG then started to operate continuously thus preventing effective communication between the Tower and any aircraft. The crew of VH-RMG attempted to call on the Surface Movement Control(SMC) frequency but the transmitter continued to operate continuously. At the same time the pilot of VH-TJQ was attempting to ascertain from the Tower if his aircraft was cleared to land. He also called on the SMC frequency but that frequency was jammed at that time by the continuous transmissions from VH-RMG. By this stage the pilot of VH-TJQ had decided to land if they did not obtain a landing clearance. When the aircraft was at about 200 feet above ground level the crew noticed that VH-HSS was lined up on the threshold of the runway. An immediate go-around was carried out. The rear section of VH-RMG still infringed the runway. During the above sequence the Tower Controller had been attempting in vain to both instruct VH-TJQ to carry out a go-around and to have VH-HSS vacate the runway. Neither aircraft had received these instructions because of the continuous transmissions from VH-RMG. The sighting of VH-HSS on the runway threshold was made more difficult for the crew of VH-TJQ by the fading light and the fact that the white aircraft was aligned with the white threshold markings. Subsequent checks of the radio systems in VH-RMG failed to find any faults with the systems.