In reply to Mossdawg:
If you want something decent and have a few hours to spare then grabbing a train and heading to lindfield would be best. There are a few places dotted around the harbour but they aren't very good (believe me i've tried!):
http://www.thecrag.com/climbing/australia/north-shore/area/12011227
The Balkans is also pretty good but takes linger to access (esp) via public transport:
http://www.thecrag.com/climbing/australia/north-west/the-balkans
Lindfield is good without a mat since it has lots of easy V0-1 problems whereas the Balkans is better for the hard steep stuff.
Have a look at other areas in the online guide and see if it appeals. You might find some interesting sounding stuff eg Vaucluse but in my experience the bouldering in Sydney is fairly limited and the sea cliffs are choss so beware!