Back in the day, the boats that were a pain to track were the SS/SSKs, Nukuler boats were decidedly easier, but they have improved the radiated noise of the SSN/SSGN/SSBNs.... Still, the thing with subs is they are disruptive even when their presence is just unknown. Between nuke and fuel cell, I'd still go conventional, but it just doesn't have the same headline appeal to pollies. Aussie needs a substantial fleet of boats, either flavor, but numbers are needed IMHO.
The “track-ability” of SSN’s has got significantly more difficult with modern reactors that use convective cooling which means the coolant pumps only operate when high power/speed is required. The Virginia class’ S9G reactor and the new RR PWR3 have this capability (not the PWR2). There are a number of other new technologies, such as Shaftless Submarine Drive (SSD) being developed so we are now getting to the stage where they are much more difficult to detect and track than conventional submarines. The USS South Dakota was built with a lot of new technologies (which aren’t discussed publicly) and, if they prove to be successful, they will be included in the Block 5 Virginia’s & subsequent submarines.
These developments are making the technology in the Astute class look outdated and the lack of vertical payload tubes means that they would not be able to keep pace with weapon & UUV developments throughout their service life.