The pilot of the outbound aircraft, VH-WJQ, reported that he had passed a single engined light aircraft which was tracking inbound at the same altitude of 7,500ft. The inbound aircraft, VH-PDF, just before the incident, was still on the area frequency and had not yet obtained a clearance into the control area. The Approach Radar Controller noticed a transponder return, indicating an aircraft inbound at 6,200ft.
There was no need to advise the pilot of the outbound aircraft as the indicated vertical separation was 1,300ft. When the inbound aircraft came onto the control frequency to ask for a clearance, it became apparent that this aircraft was also at 7,500ft. The controller realised that the transponder of VH-PDF was suspect and asked the pilot to switch the height mode off. The pilot of VH-PDF did not see the outbound aircraft.
