The pilot of VH-UBC reported at Rugby at 1725, cruising at 10,000 ft and estimating Holbrook at 1759. The pilot of VH-MJH reported at Yass at 1730 at 10,000 ft and estimating Wagga at 1755. Both aircraft were operating outside controlled airspace and the tracks of the two aircraft crossed. The two aircraft passed in close proximity. No traffic information had been passed by Melbourne flight service to either aircraft. The flight service position was manned by a trainee under the supervision of a flight service officer who was rated to operate that area. Neither person had noticed the potential conflict. The area in which the aircraft were operating has a high level of complexity for flight service. The workload at the time was assessed as moderate. Significant Factor The following factor was considered relevant to the development of the incident: 1. Neither the flight service officer nor his trainee detected a situation of potential conflict.