The trend for Disney and Universal is to go bigger not smaller. They don't want to just build parks, they want to build resorts. They want to make 100% of the revenue you spend on your holiday from hotels, food, drinks, parking, airport transfers... etc. They want to build destination parks that keep you on company property for the entire duration of your stay.
When they build parks, they're usually joint ventures that are majority owned by local governments. No Australian govt (state or federal) want one of these companies consuming 100% of an international/interstate tourist's spending. They want them to spread it around the local communities, so they will never support a large scale park in Australia.
The local population is too small to sustain a park of that scale anywhere in Australia, if it could, Dreamworld would have expanded to fill their available land and done a licensing deal with one of the major studios (like Universal).
As for international tourists, Australia is really, really far away and expensive to get to. Our nearest neighbours are generally poor/developing countries, and the ones that aren't already have Disney and/or Universal parks (Singapore, HK, China, Japan). A Korean or Vietnamese person wanting a Disney or Universal experience isn't going to come to Australia, they're going to to an Asian park or make a one-in-a-lifetime trip to the US.
I personally like that we can have parks like Dreamworld and Gumbaya (even Aussie World) that are smaller and have their own unique vibe. I'd like to encourage them to to better themed experiences and tap into unique Australian stories and experiences that you can't get in the rest of the world, rather than just cloned experiences from a park 6-10 hours flight away.