BLACK-HEADED MONITOR AND FRECKLED MONITOR Varanus tristis (Schlegel, 1839)
Both monitors are different forms of the same species. The Black-headed Monitor Varanus tristis tristis is found in drier parts of western and central Australia, while the Freckled Monitor Varanus tristis orientalis is found in north-eastern parts of Australia. Juveniles of Varanus tristis tristis often have a brownish head, similar to that of Varanus tristis orientalis. Both monitors attain between 60 and 80 cm in length, although the Black-headed Monitor Varanus tristis tristis is usually larger than its northeastern relative.
Both lizards tend to be secretive. They are mainly arboreal, except in very and areas where they tend to be found around rocky outcrops. Specimens are not often seen unless looked for, and even then they usually make for very inaccessible cover, when seen or pursued.
The diet of these lizards consists mainly of other reptiles, although other animals are sometimes eaten.

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.