Visual approaches to Sydney Airport were in progress, domestic arrivals being processed for runway 34R, and international arrivals for runway 34L. The arrival sequence was being handled by Approach South (APP-S) and Approach North (APP-N) radar controllers. Traffic density was described as light to medium.
To resolve a traffic management problem in his airspace, APP-S coordinated with APP-N for a Metro 23 to complete a right circuit for arrival to runway 34R. This involved radar vectoring the aircraft to pass overhead Sydney Airport onto a right downwind leg, east of the runway complex, an area which was normally under the jurisdiction of APP-N. However, as there was no traffic east of the runway at the time, it was agreed to keep the Metro 23 on the APP-S frequency.
A SAAB 340 (SF34) was also being processed by APP-S for a landing on runway 34R and was given radar vectors to join on left downwind. During the arrival sequence, the crews of both aircraft were given instructions to descend to 2,000 ft and were vectored onto opposing headings for their respective base legs. Some coordination concerning other aircraft took place at about this time. When the Metro 23 was passing through the final approach path of runway 34R, the crew questioned the controller as to whether they were to turn the aircraft right to intercept final approach. APP-S confirmed this, then vectored the SF34 to remain clear of the Metro 23. Recorded radar data indicated the aircraft passed within 2 NM of each other, with a minimum vertical separation of 200 ft. The required radar separation standard was 3 NM laterally or 1,000 ft vertically.
The decision to leave the Metro 23 on the APP-S frequency, whilst being vectored within APP-N, airspace was considered to be a significant factor in the development of this occurrence. Whilst the APP-S controller believed he had not forgotten to issue the crew of the Metro 23 with a clearance to turn onto final approach, it appeared he had a mind-set, or expectancy, that APP-N would process the aircraft, as would normally happen. This mind-set was experienced despite the fact that the colour of the Metro 23's radar symbol indicated it was under the jurisdiction of APP-S.