An independent report has confirmed Australian environmentalists' claims that fire has devastated Australia's tallest tree on the island of Tasmania."

Just considering the sentence on its own, it doesn’t really tell us that the tree is dead. It is devastated but, if that is the sum total of the environmentalists' claims and the other statements were made by the journalists, then it could be still alive. Many trees are quite resistant to fire I believe. Until we know the truth it is hard to suggest an alternative word to use. What kind of tree was/is it anyway? Was it Australia’s tallest tree that happen(s)ed to be on the island of Tasmania, or is/was it the tallest tree on the Australian island of Tasmania? That isn’t really truly clear either.

For a more dramatic and definite statement I suggest:

An independent report has confirmed Australian environmentalists' claims that fire has incinerated Australia's tallest tree on the island of Tasmania."