Diesel-electric sub have advantage if you want sub to do costs protection. They are more silent and can remain in position for a long time as they don't need to

Diesel-electric sub have advantage if you want sub to do costs protection. They are more silent and can remain in position for a long time as they don't need to move to cool their reactor. It means they can stay in pockets of silence longer if they need to(sub exploit variation of pressure, salt density and temperature underwater to find place where their sound waves diffuse poorly). Honestly Australia doesn't need nuclear sub. The only things they can do that diesel-electric sub couldn't is offensive missions in water further for the Australian costs. That might be useful if Australia decides to go fight in an hypothetical war against China but that would mostly serve the interest of the USA. Diesel-electric was probably a better choice because Australia would have been able to service them. As is, they will be bound to the USA.