A Boeing 767-338ER (B767) was maintaining flight level (FL) 370 and had been assigned FL330 to maintain separation with a Cessna Citation 500 (C500), maintaining FL310, that was crossing the track of the B767. The controller entered FL330 into The Australian Advanced Air Traffic System (TAAATS). He subsequently, and unintentionally, assigned the crew of the B767 descent to FL300. The controller received a cleared level adherence monitor alarm (CLAM) when the B767 descended through FL326. Vertical separation between the B767 and the C500 reduced to 700 ft, and horizontal separation reduced to 3.4 NM. The required separation standard was 2,000 ft or 5 NM. There was an infringement of separation standards.
The controller had initially cleared the crew of the B767 to descend from FL370 to FL330. The descent was to be commenced at the discretion of the crew. He then entered FL330 into TAAATS as the new cleared flight level (CFL). The controller reported that he had made the necessary TAAATS entries on receipt of the correct level read back from the crew of the B767. There were no subsequent TAAATS entries required in relation to FL330 being assigned to the crew of the B767.
The B767 crew reported leaving FL370 approximately five minutes after they had been assigned FL330. The controller reported that he had intended to confirm FL330 as the cleared flight level with the B767 crew at that time, but he unintentionally assigned FL300. The crew of the B767 read-back FL300 and continued descent through FL330. The controller did not detect from the read back that he had assigned an incorrect flight level. There were no subsequent opportunities for the controller to realise the error until the CLAM alarm from TAAATS.
The controller indicated that he considered his workload at the time of the occurrence to be light. He was responsible for two sectors of airspace but he did not believe that the increase in workload caused by the combination of the two sectors contributed to the error. The replay of the voice recording indicated that the controller had up to ten aircraft under his control at the time of the occurrence. Three of those aircraft had requested a shorter route. The controller accommodated the requests because some of those aircraft were involved in bush fire fighting operations. When the routes for those aircraft were amended, the associated flight data record in TAAATS also needed to be amended and the changes needed to be coordinated with adjacent sectors. The controller did not believe the extra workload generated by those tasks contributed to the occurrence.
The controller was also the team leader on the shift. He reported that there were no distracting team leader issues at the time of the occurrence. The controller did not believe he was fatigued.