Confidence: I'm as sure as I can be.
Notes: The lace monitor Varanus variius and sand monitor V. gouldii are the only species that occur on this area (V. prasinus occurs in the Torres Strait and on New Guinea). The habitat is not right for the sand monitor, which is found in sandy areas and does not readily climb. Lace monitors are a lot more common and are arboreal, so it is most likely to be this species. You can also make out a banded pattern from the photo which fits for a lace monitor.