The sector 12 controller was responsible for ensuring separation was maintained between the two aircraft. ZK-NZY was tracking direct from over Sydney to Melbourne and this track took it close to over Canberra. VH-EWI was proceeding via over Cooma and Canberra to Sydney. Both were cruising at flight level 280. It was necessary to retain either 1000 feet of vertical separation or five miles horizontal separation. The controller considered the option of vectoring in the vicinity of Canberra to ensure separation was maintained. However, she assessed that the lateral separtion of five miles would be retained without the need for this. In spite of this assessment the horizontal distance between the two aircraft reduced to 4.5 miles. Significant Factors The following factors were considered relevant to the development of the incident: 1 The controller did not take positive action to ensure that the minimum required separation was maintained.