I have only ever being to Melbourne once and i love it. If a disney land was built in VIC, i would definitely travel there to visit it. However, this would partially be because i have relatives in VIC who i could also see at the same time. I think climate wise, the VIC summers would be better than the QLD summers, but the QLD winters would be better than the VIC winters. I really don't see how climate effects the position of a theme park that much. I wouldn't say i'm not going to Disneyland because it's too cold. I wouldn't really care. But maybe that's just me.
If i could put Disneyland anywhere in Australia, i would choose the Gold Coast but this is mainly due to the fact that i live 2 hours away. I also think the Gold Coast is a good location because it already has existing Theme Parks that people visit. If i was an international traveller, i would much rather go to one location to visit 4 theme parks than have to go to 2 seperate locations and having to spend more money on airfares.