After all, in a war submarines are best employed against enemy merchant shipping.
If we are talking about the defense of Australia, it would seem to me that the money would be better spent on land-based aircraft.
No, no, and again NO!
Your best defense against a hostile enemy submarine fleet is enough nuclear powered subs operating on their coasts to prevent enemy subs from getting to your coast and isolating your country. You really should sink their subs in their harbors before their subs leave their harbors .... but that requires political guts to act before their subs leave their harbors. Often, this will require your action before their declaration of war . 8<)
A poor second is using your nuclear subs to intercept their subs enroute. But that's very hard.
Your next best choice is using your nuclear subs operating near your own coast to hear, intercept and sink their subs before their subs sink your merchant traffic and lay mines in your harbors.
Next best is conventional subs near your own coasts to do the same thing
But no subs can stop their spies/paramilitary/un-uniformed warriors/terrorists/Spetznaz operators from mining your harbors with fishing boats and small craft before war breaks out. Oopsie! You're dead.
“No, no, and again NO!”
And Australia still remembers Japanese mini-subs in Sydney Harbour, among other places. Australia has a territory almost the size of America’s contiguous 48 states, but without the population distributed throughout or on all coasts (in very large part due to limited water resources). Much of its economy is based on shipping its resources overseas, as well, which would be difficult to protect with landed airbases.