Excellent photo Chris. I havent seen the stripe on the back of this wallaby in other photos. Yours is by far the best photo. (Or could there be more than one such wallaby on Mt Majura? - I doubt it).
May be acromelanism as seen in 'sealpoint' siamese cats. Apparently the same genes are involved in Leucism and Acromelanism so acromelanism may be a form of leucism, or they may be different forms of the same thing.