After departing from Bankstown, the pilot of the Turbo Commander was instructed to climb to 5000 ft. At the same time a Boeing 767 was being vectored for a right downwind for runway 16R, on descent to 6000 ft. As the B767 approached that altitude the Departures (S) controller noticed that the Turbo Commander had climbed above 5000 ft and was approaching 6000 ft. The Turbo Commander was immediately turned right but lateral separation between the aircraft reduced to about 2 NM. The required separation standard was 3 NM lateral or 1000 ft vertical.
The pilot of the Turbo Commander later reported that he thought he had been cleared to 6000 ft, but may have confused the altitude clearance with a heading change to 160 degrees given at the same time.
