To answer the above, and to avoid getting into a long tangent about dinosaur intelligence (as much as I'd like to,) based on CT scans of braincases and rough brain size/body mass calculations, the majority of dinosaurs would have been roughly as smart as crocodiles. Dromaeosaurids ("raptors") and the like would have poked into the lower avian range for intelligence. Predatory group behavior is... less than clear-cut, not to put too fine a point on it.
Anyway. I don't have much knowledge on the climate of Pleistocene Australia. If it was basically the same as the present, then all you really need to do is keep humans from killing off all the megafauna (both Megalania and its food sources.) Of course, there's a good chance that people are going to get to Australia eventually; even if the First Peoples don't do it (maybe they develop a religion like the ancient Egyptians, worshiping Megalania like the Egyptians worshiped the crocodile? That would be AWESOME) then it'll be colonizers. Nobody looking to settle Australia in the modern era is going to let car-sized lizards (and marsupial lions, and giant wombats, and giant kangaroos...) run around. Unfortunately.
If Australia is like North America, and the megafauna are dependent on savannah that goes away with the demise of the ice, then game over is pretty much the only option.
There should be more religions worshiping large reptiles...