Darwin air traffic control was using runway 29 as the primary runway. Two BAe Hawk aircraft were approved to depart in the opposite direction from runway 11. A Cessna 310 (C310) had departed from runway 29 prior to the Hawks and was climbing through 3,500 ft while tracking on the 107 radial of the Darwin VOR navigation aid.
The planner controller had coordinated the clearance from runway 11 with the approach controller, who was performing both the departure and approach control functions. However, the flight strip indicated that the Hawks would depart from runway 29. As the aircraft became airborne the error was detected, but the pair were quickly closing on the C310. Traffic information was not passed to any of the crews and the aircraft came within 1.5NM of each before the Hawks had established themselves 1,000 ft above the altitude of the C310. There was an infringement of separation standards, which required a minimum of 3 NM horizontally or 1,000 ft vertically.
The investigation revealed that the planner controller had carried out the correct coordination but had incorrectly written runway 29 on the flight strip for the Hawks. When the aerodrome controller coordinated the correct information to the approach controller (using the phraseology "next runway one one"), the approach controller was not alerted by the words "one one". Consequently, the approach controller issued an instruction for the Hawks to depart on runway heading still believing them to be using runway 29.
When the pilots of the Hawks made radio contact with the approach controller after departure, the controller chose to not initiate evasive action as the rate of climb of the Hawks was far greater than that of the C310, and they had already reached the altitude of the C310. This decision was taken at a time when the horizontal separation had reduced to less than the 3 NM standard. The controller believed that the time taken to issue, and have the pilots respond to, a turn instruction would take longer to attain a horizontal standard than allowing the climb to continue until the vertical standard was achieved.