The Nullabor plains are a vast limestone area, riddled with caves. Some go for kilometres underground. And in a desert, there are many underground lakes, some of which are large bodies of water. There have also been many discoveries of fossils in these caves, animals in eras gone by which had fallen into the sinkholes, where the tops of some caves have collapsed.
The Bunyip? Thwe Bunyip is a traditional part of Aboriginal Mythology and may be a generic term for an aquatic animal that may well have existed.
You can have BigFoot, we have the Bunyip!
https://simple.wikipedia.org/wiki/Bunyip
Cheers!
Link to the book:
Desert Dragon Comic