The very place that Captain James Cook arrived in Botony Bay in 1770 is also a fantastic dive site in Sydney, probably my favourite. Set in a national park with all the colourful parrots and the laughing kookaburras keeping you company as you set up your gear it only gets better in the water. From the Monument to the Steps there are lots of colourful nudibranchs, corals, sponges, anenomes, fish, sharks, weedy seadragons, rays and even the occassional turtle.
Tide times can make a big difference at this site. When the tide is going in or out it can make this a strong drift dive and it is not pleasant fighting a current knowing it wants to take you out to sea. The entry and exit can also be difficult on a low tide or when there is high swell and so Kurnell is definitely a site for more advanced divers. There is also a lot of boat traffic.