Are you in an urban area?
I am not familiar with the habits of possums in urban areas, but in the bush they do not tend to bother young trees for food. Their natural diet is fruits and flowers. They will nibble on tasty young growing leaves. If you know they are there, then you would do well to provide them with an alternate food source. You could provide a raised platform in an older tree, well away from your new trees.
Other alternatives are catching them and releasing them elsewhere, but you will find others take their place, and the released ones will be stressed, and may die.
Cockies will do much more damage to trees. They take pleasure in snipping off young branches and leaves, just because they can.
We have plenty of possums round here, and I have never had the least bit of trouble with them (or the cockies for that matter) damaging young trees. That may be because of our dogs. The dogs are inside much of the time, but the scent of them would be around the garden.
My neighbor complains constantly about them eating her roses. I honestly never notice it, but perhaps that is just a difference in our personalities, or tolerence level, or just that I have so many roses?
The following give some suggestions for foods that possums would like. (scroll down the page for a table)

Here is a link that might be useful: possum notes

No we live in an urban area. Our suburban 7000 sqr ft block supports around 6-8 possums. They are mostly ringtail. The problem is that our neighbour has recently removed virtually all trees from their block so they can do a "vogue living" garden. Vogue living gardens usually dont have trees, just Agaves, Yuccas and flaxes.
This has displaced the possums from their land to ours. It has also taken away a lot of the screening we enjoyed.

Since next doors garden was laid waste the possums have stripped our 4 metre high avocado to within 60 cm of the ground. They never touched this tree before.

That sounds right - they are stressed as their food source has been removed. I think you will find that feeding them, or at least augmenting their food supply will help solve the problem, and won't cost you much - mostly just help you get rid of your table scraps. They are nice creatures, so long as they don't get into your roof space, and they can become quite tame. When your neighbors see you hand feeding them, they might wish they hadn't destroyed their habitat.
Perhaps you could plant a couple of trees that would provide a friendly environment for your new family, and maybe put up a coule of shelters for them?
Try and enlist the help of other wildlife friendly neighbors too. Surely there are a few people who enjoy a bit of Australian fauna in the suburbs?