FACTUAL INFORMATION The crew of a Metroliner taxiing for an Instrument Flight Rules (IFR) flight from Dubbo to Sydney, with a planned cruising level of FL190, contacted Melbourne flight service (FS) for traffic information. The Melbourne flight service officer (FSO) had flight progress strip details of an IFR Mooney M20J enroute from Gunnadah to Condobolin via Dubbo. The Mooney was cruising at 8,000 ft and its estimate for overhead Dubbo was approximately 6 minutes later. The airspace around Dubbo is outside controlled airspace and separation between aircraft is a pilot responsibility. Pilots are assisted by the provision of traffic information from FS. Outside controlled airspace IFR aircraft are provided with a traffic information service on other IFR aircraft. The Melbourne FSO believed that the Metroliner and the Mooney would not conflict due to the planned tracks of both aircraft and the altitude of the Mooney. Consequently, he did not provide traffic information to the crews of either aircraft. This assessment of no traffic confliction was incorrect and contrary to the requirements for the provision of a traffic information service detailed in the Manual of Air Traffic Services (MATS). MATS states that where aircraft are climbing through the level of another aircraft or where aircraft will have less than 1,000 ft vertical separation with less than 15 NM lateral separation at the estimated time of crossing or passing, traffic information shall be provided to aircrew. The Melbourne FSO notified Sydney FS that the Metroliner was taxiing at Dubbo and was provided with traffic information on another aircraft. The Metroliner departed Dubbo and the crew reported their departure to Melbourne FS and broadcast a departure report on the Dubbo mandatory broadcast zone (MBZ) frequency. The Melbourne FSO notified the crew of the Metroliner the traffic information provided by Sydney FS and notified Sydney FS of the departure time of the Metroliner. As the Metroliner climbed enroute, the crew observed another aircraft approaching overhead Dubbo from the northeast. The crew attempted to contact the pilot of the other aircraft via the MBZ and FS area frequencies. As the Metroliner passed 8,000 ft at approximately 5 NM from Dubbo the crew estimated they went through the level of the other aircraft. The callsign of the other aircraft was established shortly after when the crew of the Metroliner overheard the pilot of the Mooney report overhead Dubbo at 8,000 ft. The crew of the Metroliner changed to the Sydney FS frequency and confirmed the other aircraft's callsign and track details with the Sydney FSO. ANALYSIS The Melbourne FSO estimated the flight of the two aircraft would not be in conflict based on his experience and understanding of Metroliner aircraft performance. He considered the aircraft to be well separated despite the fact that the flight information on the flight progress strips indicated that the aircraft met the criteria for traffic in conflict detailed in the MATS. The FSO had recently undergone a performance check and had received favourable comment in relation to his ability in assessing conflicts between aircraft. This may have led the officer to be somewhat over-confident in his abilities and possibly caused him to trust his own instincts in preference to standard procedures. SIGNIFICANT FACTOR The Melbourne FSO did not comply with the procedures in the Manual of Air Traffic Services for the provision of traffic information to IFR category aircraft.