The aircraft had experienced APU problems during the turn-around in Sydney. This had resulted in the crew turning the APU off and isolating the system. The flight then continued, requesting ground power to be available at the ports at which the aircraft was scheduled to land.
The aircraft departed Sydney and had two very short intermittent APU fire warnings during the climb from Sydney. The fire warning light and bell did not remain on during this time. The flight continued normally until the top of descent when the APU fire warning and fire light illuminated. The crew completed the checklist actions for a fire warning and when the light remained illuminated they discharged one fire extinguisher bottle. After these actions the light still remained illuminated and the crew advised Canberra Air Traffic Control (ATC) of the problem and asked for the local fire services to be placed on stand-by. ATC declared a distress phase shortly after this notification.
The aircraft tracked for a right base for runway 35 and during the approach the warning light became intermittent. The crew advised the purser that they had a fire warning and would be landing at Canberra in 5 minutes. She was also told that the landing would be normal and the fire service vehicles would be waiting for the aircraft.
The aircraft landed normally on runway 35 and was followed by the fire tenders. The fire services advised that no fire or smoke was evident and the crew elected to taxy the aircraft to the terminal. The fire tenders again advised that there was no fire or smoke present as the aircraft approached the terminal and the crew advised that the fire services would no longer be required. The aircraft parked at the gate and the passengers disembarked normally. An inspection of the APU by engineers revealed a fault in the fire indication system.