First, I took a look at Wikipedia. It explains that the Rainbow Serpent may have been inspired by one or more of the following Australian snakes:
- The rough-scaled python
- The taipan
- The file snake
These snakes could inspire cultures across the continent. Research tells me that the rough-scaled python is active in a small area of Western Australia, though its range may be much larger. The taipan lives in central Australia as well as the coast and regions beyond. File snakes live in aquatic areas across the region. An extinct snake, Wonambia, would also have lived in the plains; other large species existed. Given the geographic spread of these snakes, they all could have inspired cultures across Australia.
I independently found one of the Wikipedia sources, which supports the conclusions (of course!), as well as a related document. This is the only other non-National Geographic source I could find. The evidence, though, stacks up to give a sensible conclusion: Snakes were everywhere in Australia, so they provided a common habitat component. And the big ones sure seemed powerful.
One issue with this hypothesis is that some of the snakes have small geographic distributions - just look at the range of the rough-scaled python! There are other species of snakes in Australia, so they could make up the difference, but the limits of the three given snakes are a tad worrisome.
A related page.