Narrabeen Lakes on the North side of Sydney is a good place to start.

www.bestsydneywalks.com/narrabeen-lagoon-trail-around-narrabeen-lakes/

http://windsurfnsnow.com.au/site/contact-page/

its too windy now , wait until people are bored at work and they will tell you all about it...
There is only one shop that might rent gear, in Collaroy it's called windsurfnsnow, they will also give you tips on the best locations.
I have a personal site where I record my sessions: martignoni.id.au/sessions.php
Click on the drop down 'Most Popular' --> Locations
then click on each location and you will be able to see a picture and some very basic info.

Give Sam or Chris a call at Windsurfsnow on Sydney (02) 9971 0999. They have gear to hire and are very close to Narrabeen Lagoon, Collaroy and Long Reef beaches.

Justin runs tours which are helpful if you don't have any gear or a car
www.windsurfingreligion.com/windsurfing-tours.html