They may have a population like QLD but most of QLD aren't Disney fans, I said they have the highest per capita fans of Disney of any country in the world. Australia doesn't.
As for the 3-4 night sailings, Aussies are used to that, most of our ships here do plenty of sailings of that length and a lot of them are sea days, no ports. Just because some people don't like to sail like that, doesn't mean no one does.
Also the rides aren't the only thing that makes a theme park, it's the entertainment, characters, theming and service. No one is expecting a few tiny rides on the top of the ship to be exactly like a theme park, but when you are this far away from a Disney park this is a fine compromise, and a heck of a lot cheaper for Aussies too.