Circumstances Canberra approach/departures control was being operated on combine due to training requirements. The training officer had confidence in the trainee's ability to handle the situation as the traffic level was light and the trainee was in his final two weeks of training prior to a rating proficiency check. VH-WNK [Cessna 210] was maintaining 9,000ft on a flight from King Island to Bankstown and was established on the 210 radial of the Canberra VOR. VH-EWF [Fokker F28] departed Canberra for Melbourne at 0906 hours and was instructed to maintain 8,000ft due to an expected confliction with VH-WNK. When the trainee controller identified VH-EWF he expected that the aircraft would climb sufficiently quickly to be able to reach a level above VH-WNK before the radar separation standard of 5nm was infringed. On this basis he climbed VH-EWF to Flight Level 190 with a requirement to report leaving 10,000ft. As the distance between the two aircraft reduced, the training officer asked the trainee what action he intended to take. The trainee was considering his reply when the training officer instructed him to turn VH-EWF so that radar separation would be maintained. At that precise moment the trainee commenced a transmission to another aircraft on an unrelated matter and a few seconds delay occurred before a radar vector was issued to VH-EWF. The training officer elected not to use the override system as he judged that the time taken to activate the system would be similar to that taken by the trainee to complete the transmission. As the crew of VH-EWF were complying with the instruction the separation reduced to 3nm before the flight paths diverged. As vertical separation had not been established, a breakdown in separation occurred. Significant Factors 1. The trainee approach/departures controller misjudged the climb performance of VH-EWF. 2. The rated approach/departures controller allowed the situation to develop to the stage where there was insufficient time to guarantee that separation standards would be maintained. 3. The training officer's override facilities were not conducive to immediate correction of a trainee's error.