The earliest disembark slot we have had is 7am, and express walk-off was at 6:45 for that cruise, however like Pacific Encounter this morning, that was a full turnaround.
Partial turnarounds run a bit later, of which we had one late last year on Majestic Princess. We were one of 400ish disembarking, and first disembark slot at 7:45am with express walk-off starting at 7:30am. Your Brisbane to Brisbane is likely a similar arrangement, with the majority of passengers doing the Sydney to Sydney lap. Given only up to 500 passengers disembarking, is also why you may not see the disembarkation tour to the airport. It might be a bit frustrating, as you can watch the planes banking and landing on your starboard side as you are approaching the port, yet they are so many hours away.