The problem is not diesel subs, per se. The problem is being silly enough to ask for nuclear subs to be converted to diesel.
It’s like asking Boeing to fit radial piston engines and propellers to Jumbo jets. Boeing will do it, for a price. A very expensive price. And the outcome would be Frankenstein’s monster.
The few submariners Australia can assemble to man them will be easy prey to predator unmanned submersibles.