Bare Island in La Perouse in the South of Sydney is a great place to snorkel. It is quite far from the ocean which means the waves are not very strong, but the current is strong enough to prevent the water to get cloudy. The snorkeling spot is inside Botany Bay, it might be possible to snorkel around Bare Island itself but I only snorkeled on the rocks between Congwong and Little congwong beach. Those rocks look safer than the ones on Bare Island itself that are more exposed to the waves and currents. Check for the jellyfish and blue bottles before you get into the water, as they can get stuck in the bay sometimes. You can see quite a good diversity of fish by snorkeling on these rocks, it is likely that you will see some Toad Fish, Grey Sweep, and Red Morwongs. If you are very lucky you might see the Red Indian Fish. Bare Island is also a great spot for scuba diving.
Compilation video of fish seen while snorkeling at Bare Island/Congwong Beach in Botany Bay:
Earth view of Bare Island in Botany Bay, the rocks between Congwong and Little Congwong are suitable for snorkeling. However this spot is probably not the best one to begin snorkeling because the waves are pushing you to the rocks.
Parking is quite easy around the area, Bare Island is a nice place to spend a day at the beach or to snorkel if you live in the South of Sydney and have a car. Be aware that it is very close to the airport so you will hear a plane from time to time: