VH-HSP departed Sydney five minutes before VH-TJP. Both aircraft were tracking via Wollongong and T18 to Eildon Weir. On first contact with sector control, VH-HSP was cleared to climb as planned to flight level (FL) 350. VH-TJP was also planned at this level but on first contact with sector control, VH-TJP was cleared to climb to amended FL 330. At this stage, VH-TJP was closing on VH-HSP at 110 knots and was 13 miles behind. A very inexperienced trainee was operating the position under the supervision of a rated controller. The controller was distracted from his supervision task for a short period and when he returned his attention to the screen he noted that VH-TJP was now only five miles behind VH-HSP and 200 feet below. The controller told the trainee to turn VH-TJP onto 150 degrees. The trainee instructed VH-TJP to turn right onto 150 degrees but immediately corrected this to turn left onto 150 degrees. The crew then queried the direction of turn. The controller took over from the trainee, told the crew to turn left and gave them traffic information on the other aircraft. Separation reduced to a minimum of three miles at the same level. Both the controller and the trainee indicated they were not very familiar with the performance of VH-HSP and that the climb performance achieved had been less than they anticipated. The controller expected that VH-HSP would reach flight level 350 before longitudinal separation was lost. Separation would then be maintained vertically which was the reason VH-TJP was assigned amended FL 330. Significant Factors The following factors were considered relevant to the development of the incident: 1. Inadequate knowledge of the performance of VH-HSP. 2. The controller did not maintain an adequate monitor of the situation to ensure that the separation strategy devised, actually achieved its aim.