Ben, did you put it in the fridge/freezer? It is definitely not just a Rattus rattus. Nor is it a swamp rat (R. lutreolus) or bush rat (R.fuscipes). My first thought was Toorrana (Mastacomys fuscus) but there is a suggestion of a bi-coloured tail (paler below) which indicates some species of Pseudomys, which have often been initially mistaken for juvenile Rattus rattus. A photo of the pads on the hind foot could help.