True about Australia having very able and fast snorkelling Collins SSKs. Australia has continuously used SSKs since 1967. Many commentators are then at a loss as to why Australia is moving to SSNs - about 5 times more expensive than the SSKs.
The answer is that nuclear subs (the world over) make the best nuclear weapon platforms... This implication, of course, contradicts the Australian Government's spin that our SSNs will be a curiously extravagant way to fight a conventional war.
Australia needs a deterrent against China. Australia projecting even conventional explosives against the Chinese mainland would likely mean a Chinese nuclear response. Australia, against China, has a need for long range strike weapons - with nuclear subs being the most discrete way to move close to the Chinese coast. This will put China's population centres at risk - thereby improving Australia's essential deterrence strategy.
Australia's use of nuclear explosives won't happen in a broader regional proliferation vacuum. It is likely Australia would find justification moving to nuclear explosives after South Korea and Japan have finalised nuclear weapon capabilities (even short of these countries having such weapons). Also the unknown India factor may play a part if India's power projection in the Indian and Southern oceans become somewhere between ambiguous and threatening. What Indonesia does or may likely do might also be a factor.
If the US extends long range nuclear sharing to Australia - something only extended to short range NATO country strike - might be something that heads off Australia needing to develop a complete national nuclear capability.
I have created the acronym SSHN for Submarine, Nuclear Powered with Hypersonic Missiles. Advanced missiles, with hypersonic glide vehicles. These would make even Australian SSHNs (like Virginias (if ever delivered) or then SSN-AUKUSs) a more cost-effective platform than traditional large SSBNs which would be too expensive for Australia - a middle power. The "B" for Ballistic missiles (SLBMs on submarines) are also proving relatively easy to shoot down in the Ukraine and especially in the latest Israel vs Iran conflict. SSHNs can accommodate smaller intermediate range 3,000-5,500km missiles - missiles with a shorter flight-warning time than full-size SLBMs, like the 12,000+km range Trident D5.
So Australia's long held resistance to all things nuclear is being overcome by fear of China - triggering the need, or option, of having Australian nuclear weapons. Alternate Australian nuclear weapon platforms, be they long range bombers or land based ICBMs in silos, are too specialised. Such bombers and ICBMs are also very vulnerable to pre-emptive strike from Chinese submarines that might launch hypersonic or ballistic missiles off Australia's coast.