First find the entry point Then get some Camphor Laurel moth...

First find the entry point

Then get some Camphor Laurel moth balls. preferably wait until a warm day (if you are in Melbourne that could be a long time) the open the access into the roof and thrown a goodish number of them around the roof cavity

The mothballs give off an intense odour when the roof gets hot and the possums can't stand it so they leave, then block their access point after they have left

If using a possum/cat trap try a little peanut butter on the apple, they are unable to resist it