So if an Australian ad tells something is "the best" or something similar and you can prove it isn't, you can get your money back?

Usually subjective opinion isn't binding (though I'm sure there are exceptions to this across jurisdictions)

The link I shared makes it quite clear that "puffery" that nobody is reasonably expected to take literally does not count.

Being told that the app you paid for would be a one-time payment, and then having the service deliberately degraded to try and force you into a subscription model, is clearly not puffery.