The southbound aircraft, BA 11, was cruising at FL330. The northbound aircraft, MH 24, had been cleared to climb to and cruise at Flight Level(FL) 350, with a requirement to reach the assigned level by 1801 hours local. Shortly afterwards BA 11 requested a diversion to the right of track. The aircraft was cleared to divert up to 90 kilometres to the right of track. The tracks flown by the aircraft were laterally separated by approximately 80 kilometres. About five minutes after being given approval to divert from his cleared track, the pilot of BA 11 requested a clearance to climb to FL 350. This request was denied by the Perth controller and the aircraft was given approval to divert either to the left or right of track, as required. At 1811 hours local the Perth controller gave BA 11 a clearance to climb to FL 350, at 1816 hours local. The aircraft acknowledged the clearance and during the subsequent climb advised sighting another aircraft, MH 24, an estimated two kilometres to the right and 1400 feet above his level. The investigation determined that the Perth controller had mistaken the time MH 24 had been required to reach FL350, 1801 hours, for the time he had calculated that the aircraft would pass. He had thus applied the required separation standard to this time instead of the calculated time of passing of 1818 hours local.
