This isn't part of the NPT - this is a tacit agreement that nuclear nations don't sell nuclear (or nuclear powered) weapons to non nuclear nations. This is why North Korea never managed to buy nuclear stuff from its Chinese ally. So far.

There has never been such an agreement.

What there has been is the NPT's Article I prohibition on the provision of nuclear weapons or nuclear explosive devices to non-weapons-states (which are all but the 5 weapons states, USA, Russia, France, UK, China). It also prohibits assistance / inducement in developing weapons.

The "(or nuclear powered)" bit has never been a thing.

There is also the Nuclear Suppliers Group, which is a group of nuclear-technology-exporting countries that agree not to export to NPT non-weapons-states a much longer list of dual-use nuclear technologies, unless IAEA comprehensive safeguards agreements are in place. Australia is a member of the NSG, and has safeguards agreements with the IAEA.

The IAEA has been informed by all three states of this new proposal.: https://www.iaea.org/newscenter/pressreleases/iaea-on-trilat...

A submarine is not a nuclear powered weapon, or even a weapon unless they’re going to be using them to ram adversaries