Nuclear submarines are for projecting power
globally. They have incredible range, and can stay submerged longer. They are a
strategic weapons platform.
A diesel sub is faster, quieter, more manoeuvrable, and of course cheaper. It has a smaller range. Diesel subs are used to project power
locally. A diesel sub will destroy any nuclear sub. They are a
tactical weapons platform.
People tend to equate, after a few moment's thought, that a nuclear sub must be better than a diesel sub because it is nuclear. This is incorrect.
Australia, because it is not patrolling
the entire ****ing world needs diesel submarines to protect its supply routes from any current, or future adversaries. Diesel subs are far more appropriate for this task; protecting a fleet; fighting.
Our major trading partner is China. The most likely future adversary is China. So, to protect our trade routes with China, from China, we need all these subs.

(personally I think China is just buying Australia)