I don't want to seem too negative but Sydney Aquarium is now a Sealife Aquarium so I wouldn't expect too many unusual species. I visited most recently about 3 years ago (Australia has had a lot of state border closures in the meantime) and even then the Sealife-ification was well under way.
I know, it's a shame. Haven't visited the aquarium since 2008, but I'm expecting it to be less interesting now, except for the dugongs which replaced the sea lions and fur seals.
It's not just Sydney Aquarium. I don't know if you visited Melbourne Aquarium when you were in Australia but around that time the centrepiece oceanarium was based on the southern ocean and displayed some unique species. Nowadays the aquarium is a replica of any other Sealife and I haven't visited in a few years.
I did not visit Melbourne, but I'm sad to hear about that. If you visited one SeaLife, you kinda visited them all. I've seen 3 in Europe, and I don't feel like I need to see the rest of them.
I reckon the aussie sea lifes are worth a visit, they do have some cool species. Melbourne has Spotted Handfish and Speartooth Shark, Sydney has the Dugong and hagfish, and Gold Coast used to have a large exotic freshwater collection - quite rare in Australia! (it has since been replaced by Fairy Penguins)
