They do moult to a degree between winter and summer but they keep a thick coat right through summer too and can handle pretty high temps. If there is a hair out of place on a brushtail, something has happened. That's pretty extensive hair loss on the rump, you can see skin. There are only a few possible reasons - they have had to escape from something and wedged themselves in a fork in a tree for 1 to 2 days, they have dropped fur in defence if something has chased them, or they have stress dermatitis which happens over time - which is what I think it is a mild case of. Can't be sure but I'm thinking its a female from what I can see, especially the way the belly is hanging down, makes me think there has been babies before or possibly one on board and there is no stain on the chest from scent marking. The shape of the face seems female - bit uncertain about those strapping shoulders, that's fairly male. Females tend to stay put unless they have a reason to move, whereas the young males wander a lot. He/She is very healthy - nothing to worry about. It's all conjecture without knowing what the possums been up to these last few weeks. Of course, if you spot a large pair of balls next time you see this possum, you will know :p