VH-ANJ was tracking from over Canberra to Sydney at flight level 150. VH-WGW was tracking outbound from Sydney via Pager, Shelleys and over Canberra, climbing to flight level 140. VH-ANJ was turned right onto a heading of 050, for positioning on a 15 mile final approach for runway 34 at Sydney, and cleared to descend to flight level 130. This heading change put the two aircraft on opposing tracks. To maintain separation, VH-WGW was turned to the right 20 degrees. At that time the two aircraft were nose to nose and 15 miles apart with a closing speed of 450 knots. VH-ANJ was then turned a further 10 degrees to the right, however separation reduced to 3.5 miles laterally and 700 feet vertically as the two aircraft passed. The minimum separation required was five miles horizontal or 1000 feet vertical. Significant Factors The following factors were considered relevant to the development of the incident: 1 The controller did not take the appropriate action to ensure adequate separation was maintained.