I have another sighting of an orb today on the track leading north from Turingal Head, Bournda NP. Today's sighting I have tried to capture from different angles and will check them as soon as possible but I am catching up with a few other days of hiking/photos.
100% Nephila edulis is my two bobs worth. When you consider how common this species is just a tad inland from you, all the way down the east coast and then add the fact that they distribute via ballooning (some travelling 10s or 100s or even 1000s of kms) then it is not surprising at all that you found it in this location. For the (annual) record, N. edulis have been in very low numbers in the Canberra area this year. A dry summer may have something to do with it?