FACTUAL INFORMATION The Williamtown approach radar (APPR) controller contacted the Brisbane Sector 15W controller and advised that track shortening for northbound aircraft was available through military airspace. The APPR controller used ambiguous phraseology which left it unclear as to the terms of the clearance provided to Sector 15W for aircraft tracking via Williamtown airspace to Coolangatta. The Sector 15W controller believed he had been issued a clearance for any aircraft to track via the airspace, with a requirement to pass each relevant aircraft's radar identification to the APPR controller. The APPR controller understood that the Brisbane controller could expect a clearance via military airspace after an aircraft's radar identification was relayed to him. Neither controller attempted to clarify the situation during the period prior to the incident. The Sector 15W controller conducted his own co-ordination as the planner position was not operating. The Sector 15W controller passed a number of aircraft radar identifications with flight details to the APPR controller. These aircraft radar identifications had been passed to the APPR controller before the aircraft entered Williamtown airspace. On each occasion the APPR controller advised concurrence with, and read back, the tracking and level details of each aircraft. The Sector 15W controller became busy with a number of aircraft and subsequent co-ordination and did not pass the radar identification of a northbound B737, on climb to flight level (FL) 330, as the aircraft approached Williamtown airspace. The Sector 15W controller was also briefing another controller with the intention of opening the planner position. The APPR controller contacted the Sector 15W controller and requested departure instructions for an aircraft at Williamtown about to depart via Sector 15W airspace. While the two controllers were discussing the availability of departure instructions for this aircraft, the APPR controller observed an unidentified aircraft overhead Williamtown and requested details from the Sector 15W controller. The Sector 15W controller advised that the aircraft was a B737 at flight level (FL) 330 tracking direct to a position near Coolangatta. The APPR controller considered there had been a breakdown in procedures but did not discuss the perceived breakdown with the Sector 15W controller. The Sector 15W controller believed he was operating in accordance with the co-ordinated arrangements and continued at the position until the completion of the shift. There was no breakdown in separation. ANALYSIS The APPR controller could have used a number of specific terms to grant approval for the transit of Sector 15W's aircraft through military airspace. Any of these terms would have explicitly indicated the status of the approval. The Sector 15W controller could have been more vigilant when he accepted the offer of the use of the airspace. Possibly, in his haste to utilise the airspace he did not appreciate the ambiguity of the phraseology. The use of imprecise and ambiguous co-ordination phraseology enabled a situation to develop which could have had more serious consequences. SIGNIFICANT FACTORS 1. Neither controller considered the ambiguous aspects of the non-standard co-ordination phraseology. 2. The Sector 15W controller did not pass the radar identification of the B737 to the APPR controller in a timely manner.