Welcome, Al!
I found a website dealing with this:

www.low-impact.net/index.php/20090801/protecting_vegetable_garden_from_possums/

As far as I can tell, all he has done is built cages around his vegetables. Probably good for herbs, but a bit impractical for trees! Another possibility is getting a product called Critter Out (www.deerout.com) since that repels the American opossum (also a marsupial), but I have no idea if that's available in Australia. Making a spray of 20 ml chili powder, 3ml olive oil, 9 ml dish soap, and 1 l water may help, since we use it for rodents and rabbits here in the Arizona desert, but I'm uncertain how possums might respond to that, and it probably wouldn't repel them from the railings (unless they like to chew on them). This piques my curiosity: I've dealt with everything from mice to wild peccaries, but this is new to me! I'll keep searching.

Thank you for your ideas. regards Allan al1348

Quite welcome, Allan.