About an hour south of Sydney, and not like most people's preconceived ideas of a botanical garden. You need walking boots and a car or bicycle to do the place justice - it's enormous. They give you a map at the gate when you pay your entrance fee. Say thank you but regard it as a pretty and imaginative souvenir rather than a helpful guide. The map on their website is better, it's the right way up for a start, but still shows only some of the walking tracks, and fails to mention that most of the roads are one-way.
See: http://www.rbgsyd.nsw.gov.au/mount_annan_botanic_garden/visitor_information/map_and_photos

July 27, 2008: Visited again. Given new map at gate, much better than old map, but it still doesn't mention that the roads are one-way, obviously designed to make you drive around in circles trying to find the exit.