For those not familiar with Melbourne, Carlton Gardens is a park and World Heritage Site on the northern edge of downtown Melbourne, home to the Melbourne Museum and the Royal Exhibition Building. It also has a thriving population of brushtail possums (Trichosurus vulpecula) - but while they aren't uncommon, I've never seen them gathering in numbers like this. I counted at least twenty around the base of this tree, with more up the tree, and similar groups around other trees I could see.
My hypothesis was that there could have been a reproductively receptive female up the tree, and the males were competing for the opportunity to climb up and join her, but this is pure speculation on my part. The possums on the ground seemed to show no hesitation in running up to me, and at times acted quite aggressive, but whether this is some kind of territoriality to do with mating, or just that being inner-city animals they're accustomed to humans, I don't know.
Whatever was really going on, I at least got the chance to observe the possums up close, and take a few decent photos.