I wondered about it during my visit to Rockdale library, but here I have conclusive proof of the Sydney Subject Specialisation Scheme (or SSSS – what an acronym), a scheme in which Sydney public libraries would specialise in a particular subject, the idea being that, across the Sydney metropolitan library network, there was held a comprehensive collection, which could then be accessed by interlibrary loan. While the scheme has been discontinued, these areas of specialisation can still be detected in library collections. How the topics were divided up, I’m not sure. Hornsby, for example, collected The Bible, Linguistics and Cookery.
This information comes from “Sydney Inside Info”, an alternative guide book to “help you get Sydney before it gets you”, published in 1979.
More comprehensive lists can be found here along with everything you might ever need to know about inter-library loans in NSW.