A bird? A plane?

No, it's a bat. A megabat in fact, also known as a flying fox or fruit bat. They are native to Australia, but I have never seen one in Blackheath before. I would be interested to know if other Blackheath Blippers have. Apparently they do turn up at this time of year to attack the fruit trees (if the birds leave any for them).
When I first saw it I thought someone had chucked their jogger across the power line but it's a bat alright. A big one too, at least 50-60cm long. It didn't move at all but I presume it was still alive.
I find them rather scary as they can carry several diseases fatal to humans including Hendra virus and the rabies like Lyssavirus. So look in LARGE without getting too close.