I can't find the video now, but I'm pretty sure Marles or someone from Australia defense said something alone the line of they don't know if there was a sub in the task force.
I think it is likely that PLAN had sent a 093A along with the fleet.
This is kind of interesting to me. When facing a middle power like Australia, the importance of nuclear sub really shows up, because 055 essentially protects sub from having frigates and MPAs getting really close. If diesel sub is nowhere nearby, which is likely the case here with Collins class, there is not much Australian navy can do to actually get close enough to hunt down a moderately quiet nuclear submarine.
After all, not every navy has PLAN level of sensor density in ECS and SCS. Indonesia is not going to allow America to build SOSUS along its islands (at least to the best of my knowledge).
I wouldn't know enough about USN operations to know if they would be tailing a 093A leaving SCS. It's quite possible they would, but also possible that they stay far enough from sensor dense zone that they would not be tailing one.
But as 093Bs get added into the fleet, it seems to me that they can probably go solo into the water around Australia all the time and do whatever reconnaissance they need to do.