I was up on Sydneys Northern Beaches two weeks ago it was great diving! I just googled Bronte Beach and it looks good man! Just using the Google Earth view you can see reef south of Bronte Beach along Calga reserve and north of that it has Marks Park which looks like it's even better
Just check that you're allowed to spearfish there I'm not sure rules or restrictions for where you're going
Someone also told me that people don't dive in Sydney Harbor because of bull sharks not sure how accurate that is but I myself would avoid it anyway haha. =\
have fun man show us some pics after!