I'm no expert but I have been to the area from under the Harbour Bridge, through Circular Quay to the Botanic Gardens and up to the Opera House. Not sure if there are places with better "biomes" but there aint no better for pokestop density (in Aus). Because it's such a touristy spot there seem to be pretty decent spawns happening regularly but since the tracker will only show you pokemon that are basically right next to you, it's easy to miss the good stuff.
Not worth playing with gyms in this area as the turnover is insane.
It's very busy, lots of people, I can think of more pleasant places to pogo, though as a one-off it's fun.