Thanks Episcophagus, The thing is the 3 skulls are all from the same area, on the western side of the range near the Star River which is just West of Townsville along the Gregory Development Road. The two most common Kangaroos in this area are the Eastern Grey Kangaroo and the Common Walleroo. I believe number 3 is an Eastern Grey but this is based purely on size, it is noticably bigger than skulls 1 and 2.
The comparison of the skulls, particularly from above, shows clear differences and it is this which is puzzling, the lack of reference books on the subject does not help.
I have narrowed it down to 4 possible animals Red Kangaroo, Eastern Grey Kangaroo, Common Walleroo or Antilopine Wallaby. There are two other possibilities the Agile Wallaby and the Whiptail Wallaby Both are smaller and have a more pointy face than the animals stated earlier so i have discounted them as highly improbable.
The issue is that Red Kangaroos and Antilopine Wallabys are rare this close to the coast and if these are skulls from one of those animals which is which?
The mystery deepens.