A Cessna 152, VH-XPG, was sequenced for landing on runway 12 at the same time as a Boeing 747, VH-EBY, was sequenced for runway 15. A separation breakdown occurred when both aircraft converged towards their respective runways and the Cessna 152 was instructed to go around. The approach radar controller had previously co-ordinated the landing of XPG on the non-duty runway 12. The aircraft's position in the landing sequence had not been determined, but would be close to that of EBY. From the aerodrome controller (ADC) position, XPG was radar observed just inside 4 NM final for runway 12 as the pilot of EBY contacted the tower at 6 NM final for runway 15. XPG was not on tower frequency; the expectation being that the aircraft would sequenced (by approach) to land behind EBY. With EBY at approximately 2 NM final, XPG was continuing the approach at approximately 1 NM final and the pilot was not yet in contact on the tower frequency. An attempt was made to establish whether the pilot of XPG was on the tower frequency, and this proved negative. Instructions were then issued to the approach controller to send XPG around immediately. Traffic advice was issued to the pilot of EBY as the pilot of XPG made the missed approach. The approach controller did not ensure that separation was maintained between EBY and XPG when the latter was cleared to leave the western VFR route on a visual approach for runway 12. The approach controller should have arranged for the provision of visual separation by the ADC or issued instructions to establish and maintain lateral separation between the two aircraft prior to radar separation reducing to less than the standard of 3 NM. The controller did not appreciate the implications for maintenance of separation when he cleared the pilot of XPG to track for final for runway 12 and to make a visual approach. The controller became distracted with other traffic considerations and did not maintain an adequate scan of the radar display. The flow controller had been advised of the inclusion of XPG in the traffic sequence. The flow controller was satisfied that XPG could be included in the traffic sequence without detriment to other aircraft; subject to the approach controller providing final sequencing. The approach controller did not ensure that adequate separation and sequencing was applied between the two aircraft. Cairns ATS is developing amended flow/sequencing instructions which will include procedures for aircraft arriving to either runway 15 or12, or the runways concurrently.