Wait until spring when there is a big litter of little possums and possum crap all over. I would move them to a new home if nothing else.
As I recall from a visit to New Zealand the possums in Australia are a protected species and are not related to the North American Opossum. They are more like a raccoon. Where as in NZ where they were imported as fur-bearers they are a destructive nuisance and farmers often pay hunters to remove them from their orchards...at night. Their eyes glow red when you spot them with a flash light.