It may be a huge swan nest, with the apparent entrance being a shadow. However, megafauna such as huge swans are no longer found in Australia. I meant that this may be a huge nest made by a swan. I will post it to the COG chatline to confirm because several birdos visit the site regularly.
Swan nest in that location was confirmed by Julie Clark of Canberra Ornithologist's Group (COG) who is a regular visitor. Apparently the pair had repeated attempts in the same location, possibly explaining the height of this particular nest.