RIDGE-TAILED MONITOR Varanus acanthurus Boulenger, 1898
This lizard is common thoughout drier parts of the northern half of Australia. It attains 60 cm.
The Ridge-tailed Monitor is found in a variety of habitats, but is most common in rocky locations. It hides in rock crevices where its tail, which has circular ridges along it, will make it hard for specimens to be dislodged. In the Pilbara region of Western Australia, I have caught adults inside termite mounds during the day, in both rocky and flat areas.
Apparently this monitor prefers to feed on smaller lizards, mainly geckoes and skinks, but young specimens are presumed to be mainly insectivorous.

The above was from the book Australian Reptiles and Frogs by Raymond Hoser and now available on a fantastic CD-Rom along with a vast amount of other information, papers and the like on reptiles, frogs and other wildlife.