As I’ve said before, the possums have definitely raided our lemon tree. Yet, other people have citrus trees, and are seemingly unaffected. Are out lemons better than those of our neighbours?
Another theory was offered by a family member. Apparently ring tail possums (the variety we have around here) prefer not to walk on the ground. So they stick to fences and the like. And, since our lemon tree is next to the fence, they can access it it easily.

This is broadly consistent with what is said here, (about them being vulnerable on the ground) and is certainly in line with my experience. Maybe I’ll try and trim our tree so that it’s away from the fence and see if it makes any difference.