I guess it's like God then. You can know for yourself whether God exists, but you cannot tell others, just as with Australia.
Then again, if we wanted we could reduce all of human experience to that basic principal. Every human experience is unique, therefore there is no universal experience, therefore every 'Australia' that man thinks of, percieves, or even steps foot on would be different from every other 'Australia' no two would be perfectly identical.
However, as men, we must gurantee our survival, and in order to do this, we must utilise each others specialities through mutual cooperation. To do this, I take some of what you say as truth, whether all or none depends on a few factors, namely, whether or not I trust you, whether your experience are close enough to mine, and whether or not I am gullible or cynical. However, when multiple people state the same thing, and they are trustworthy, then the thing is generally accepted.
For example when I step foot in Australia, I may say 'The sky is blue here like it is elsewhere, therefore this is like other places I have been, and comfortable.' But another man may say 'The earth here is red unlike it is elsewhere, therefore this place is unlike other places I have been, and alien.'
Both of us are true and correct, we have both drawn accurate conclusions, and we are both correct, although the conclusions may be different.
This is not the main point however, which is coming up, I assure you.
Though we may percieve things differently, we all share several general conclusions that differ only among singular populations, (whose sanity or intelligence is often judged to be lacking.) These conclusions include most mathematics, the proper uses of language, and (in most cases) historical fact. However, there are exceptions. Sometimes history will be recorded erroneously, sometimes mathematical equations will go decades before an error is found and corrected, and languages invariably change with time. So, how do we keep a firm view of what is 'human knowledge' that is, what is our method for determining what is true concerning things that we have not witnessed ourselves? Typically, the view is to accept from an authority, but then again, an authority is a singular individual, and capable of error. However, when a thing has been witnessed by a mass of people, it is hard to simply throw it out as a falsity or myth.
We therefore shape our minds from two sources, public opinion, and authority. Personally, I am disgusted by the 'authority' present in our world today, however, I still have hope for the public. People (at least some of them) are still capable of independent thought, and it is these few who hold the future. However, this post is not about the future, or the nature of modern authority, rather it is about a very real and very probable Australia.
It is probable because of the vast majority of people who claim it is real. It would be erroneous to compare the amount of visitors to Australia to the amount of visitors to space. There are far less people who must be convinced regarding space, as opposed to Australia. And even then, 'space' is a hard to define term. Most flat-earthers seem to think that if man were to make it into orbit, the flat-earth theory would be ruined, but this is not so. Perhaps there are further complications with the theory? Occam's Razor and Russel's Teapot try to keep us from expanding theories, and to limit us to only probable conlcusions, but I htink it should be noted that at one point 'round earth theory' would be considered superfluous.
So, does Australia exist? I do not know, I suspect that it does, although I have not been able to confirm this. My main point is that to compare Flat-Earthism with Australism would be erroneous.
If you actually read all of this, I commend you, I only read it once through to proofread, and probably missed quite a bit.