Trick might be finding one.
Indeed. The modern boats are all needed at home otherwise they wouldn't be being built. A clapped out, end-of-life boat is hardly representative of what Australia will eventually operate and SSNs go out of service for good reason. Better to have a healthy-sized Australian seedcorn contingent in both US & UK training pipelines for the next 10 years (or thereabouts) than any Australian training boat. By then, the fleet-size issue should have eased for both would-be suppliers and the Australians can purchase an early-production Astute or 774 that'll at least be only one generation shy of the boat they'll be fielding. If they do get a T-boat or 688 soon-ish, I have doubts it will still be in service for HMAS Nautilus to supplant. The US & UK have the training establishments. Use them to full advantage!
Or they could buy a Han?