Hi Michael and natureguy, Black Rat Rattus rattus is hard to separate from the native Bush Rat Rattus fuscipes. In this case its a bit easier than many but the best identifier is the relative tail length. In Rattus rattus the tail reaches beyond the snout, so including the whole tail in the photo is a good idea.
With a captive specimen, you can also use smell to narrow the field - R rattus consistently produces an unpleasant pungent aroma; R fuscipes often has a sweet smell.