VH-RJE was tracking inbound on a VFR flight from Rockhampton via Charon Point (CHP), on a bearing of 143 degrees from Mackay. When the aircraft was about 35 NM from Mackay, the Brisbane Sector 7 controller provided information on the flight to the Mackay tower co-ordinator. When the co-ordinator created a flight progress strip for the aircraft the tracking details were incorrectly inserted as being via Shute Harbour, a bearing of 328 degrees from Mackay. This information was available to the aerodrome controller when the aircraft reported at 33 NM from Mackay at 4,500 ft. No other position information was provided by the pilot, nor did the controller request any. A clearance to enter controlled airspace and to track direct to Mackay at 4,500 ft was issued.
VH-TNU taxied at Mackay for Rockhampton and was cleared to track direct to Rockhampton, on the 143 radial. VH-RJE requested descent and was cleared to 3,000 ft. VH-TNU reported having left 1,700 ft on climb to FL150 and was transferred to the sector 7 frequency. VH-RJE was then asked to report crossing the coast. (The aerodrome controller expected the aircraft to cross the coast about 12 NM from Mackay on the Shute Harbour track.) The pilot reported that he was over land and approaching Hay Point (about 9 NM SE of Mackay). After the controller confirmed this position he immediately informed the sector 7 controller of the confliction with VH-TNU.
In the meantime, the sector 7 controller had observed VH-RJE on radar and was seeking confirmation of that aircraft's altitude.
By the time VH-TNU had passed 4,000 ft on climb, lateral separation between it and VH-RJE was 4 NM. Although VH-RJE had been displayed on radar to the sector 7 controller prior to this time, it had not featured in the controller's planning as it was outside controlled airspace.
For aircraft flying between Rockhampton and Mackay, laterally separated tracks had been developed. VH-TNU had planned via the western route but was given the direct track. VH-RJE had planned and tracked via the direct track.