When the pilot of VH-MXL lodged his flight plan, relevant details were inadvertently not transmitted to all necessary recipients by the AFTN system. As a result, Melbourne Approach was unaware of the planned track of the aircraft. The flight evidently proceeded normally until the pilot had reported his position at Port Phillip Heads to Melbourne Flight Service, and had requested an airways clearance. Essendon aerodrome was closed to Visual Flight Rules operations, and Melbourne Approach, after receiving details from Flight Service, cleared the aircraft to enter controlled airspace at 4000 feet on track from Port Phillip Heads to Essendon. This clearance was passed to the aircraft 11 minutes after the initial request. The Approach controller had not been informed that the aircraft was tracking via Frankston, a track which crossed the NDB instrument letdown pattern for Moorabbin. Shortly afterwards the aircraft was identified on radar by the Approach controller, and it became evident that the aircraft was in conflict with VH-TKM, which was conducting an instrument approach to Moorabbin. The airways clearance for VH-MXL was cancelled, but before positive action could be taken the aircraft passed within about half a kilometre of each other. It was considered that a number of communication breakdowns contributed to this incident. (a) The flight plan was incorrectly addressed. (b) The relevant Flight Service officer did not inform Melbourne Approach of the tracking details of the aircraft. (c) The Flight Service officer did not query the clearance as issued, although he was aware of the tracking details. (d) The pilot did not query the clearance, although it would have been virtually impossible for him to comply with the tracking instructions from his present position.