Why does Australia need a nuclear submarine?

It doesn’t even matter.
The threat of having (to deal with) a nuclear submarine should do the trick.
They’re are in pact, that’s what is the big thing.

Nuclear refers to the power source not what it is armed with. Currently Australia has diesel subs. Nuclear subs can stay submerged longer and have greater range (because you don’t have to refuel them) etc…

Because Australia along with the U.S. is preparing for potential war with China. Diverting that potential event is a top priority. The only thing keeping China from taking over Taiwan and then eventually all of Asia are consequences. After what the U.S. did in exiting Afghanistan the likelihood has increased.

It is merely a choice of upgrading to a new sub with nuclear power source vs fossil fuel.

Cold Fusion cars are next.

A nuclear powered sub can stay under water for a long time and can travel great distances. The alternative, diesel electric,
is severely limited by the need for diesel fuel, intake air, and combustion exhaust elimination.
A sub’s key feature is stealth. A nuke can hide much better and longer than a diesel electric.

It is no secret that major powers are working on technologies to find enemy subs. There are self powered sensors that can sit for decades waiting for a certain signature from a sub and then sending a signal home with the info. The signature can be acoustic, thermal, nanovolt electrical field, magnetic, light, even small water current anomolies.
Some of these advances are making their way into the medical health realm.

@Blackwater_Park This. It’s part of an ongoing cold war with China that is being ramped up in wake of the Afghanistan withdrawal. Australia is in a strategic location.

A nuclear submarine doesn’t mean it carries nuclear missiles. It means it is powered by nuclear power. @LuckyGuy answered the question perfectly.

The AUKUS deal has agreed to furnish Oz with nuclear submarines but not with nuclear weapons.

Unless there is a war with other countries who have nuclear weapons it is extremely unlikely that Australia will ever be supplied with nuclear weapons.

In the event that the subs supplied to Australia are armed it is more likely that the UK will supply them as Oz is a member of the British Commonwealth but of course it is possible that the US could also supply them.

Or it could have a deal the same as the Netherlands do.
We have nuclear warheads, but we don’t have nuclear warheads.

The Australian Defence budget explained in full