There were two main species of possum that we had on our property in Tasmania, the grey Ringtail Possum (Pseudocheirus peregrinus), and the darker and slightly larger Brushtail Possum (Trichosurus vulpecula). The Tasmanian brushtails that we have seen are usually a dark brown, almost black-looking animal, whereas mainland ones can be more of a grey colour.
Possums are generally nocturnal, but as in this photo taken in June 2010, you can sometimes see them during daylight hours. This one was part way up one of the eucalypt trees (probably a Stringybark species) on our property and was keeping a watchful eye on my dog and I.