G'day Emma,
If your local 'National Park' type people will not help, ( try WIRES), then
I would deal with the plum tree. They,(possums), feel at home in the tree
because they know that once a year they beat the local bats to most of the
fruit. Maybe it is time for you to give that tree a serious pruning, and set
it up for some nice new growth for next year, (I'll bet they got all last
years fruit)!

China
Wingham
NSW