Hi Wen
When I used to visit Sydney I hired gear from Balmoral Sailing Club (as alluded to earlier). There is not a huge selection of gear, and it's pretty 'well-used', but it might still be your best bet. Give them a call first to check they're still doing it though.
www.sailingschool.com.au/The bay is sheltered by big hills to the south, so don't bother unless there's a N - NE wind forecast. You can get a bus from the city to Mosman and walk down to the beach from there.
Hope you manage to get a sail in