Hi Chris @cmobbs, Tail of introduced Black (Roof) Rat is longer than its head+body. Tail of native Bush Rat (R fuscipes) is shorter. Also, R.fuscipes has not been recorded in Canberra or Canberra Nature Park for many decades, if ever. And R. norvegicus is rare here. Our only native rat in Canberra is the Rakali.
In the mountains there are four rat species, R.rattus, R.fuscipes, Mastacomys fuscus and Rakali. Also the extremely rare Smoky Mouse is the size of a juvenile rat.
Here is a link to the CNM advice about identifying and managing small mammals, titled 'PHOTOGRAPHY OF SMALL MAMMALS FOR IDENTIFICATION' Rodents