No, it wasn't visible to the public.
Go have a look where the venue is (right next to railway line on Harbour Bridge at The Rocks). Can't get more secluded.
Go look where the private terrace is (3 stories up, tucked away with no side or rear access).
Put simply, there is absolutely no way these photos were taken from a standing position. Standing on a 4WD possibly (at a pinch), but most likely taken from a hired scissor lift with a 200+mm telephoto zoom.
In other words, this was pre-medidated by the media. As for how they knew, probably staff / a staff member tipped the media off that the Bulldogs would be there.