Australia does exist, but not in the form you know it. It preiodically sinks below the waves.
This is why no one discovered for so long, dispite the fact the Americas were further away and they still got found.
The reason for the conspiricy is because the British Empire made a lot of money from taxing imigrants to Australia, so after it re-sunk, they had to pretend it exsisted to keep poeple paying to go there.