If you want to get technical the constitution has been changed many times without a referendum outside of the section you quote, that says how it can be changed.

Many sections of the original constitution had the words, untill the parliament says otherwise.

The parliament did say otherwise and the constitution was therefore changed without a referendum. Multiple times in multiple places.

Also ther was never a referendum in the late 1800s to decide if the wording of the constitution was acceptable to all people at the time.

Going back again it was a ACT of the UK parliament the gave the constitution authority. That act was passed by people that did not even live in the Australian colonies.

And what gave the UK parliament the authority to make laws. The Magna Carta. Supposedly it documented the movement of authority from the royalty to the Parliament. But what gave It authority. The Royalty. Who gave them authority. And so it goes on.

History is littered with authority that because wars and many other thing now no longer exist.

Ther are only 2 things I can think of that keep law and constitutions in existence. Oppression and aggregate faith.