There are several useful websites on the sinking of HMAS Sydney:
The National Archives of Australia’s Fact sheet 111 provides information on records held at the archives as well as links to other resources and guides. For a comprehensive guide to government records, see the following book in the Library's collection: The Sinking of HMAS Sydney: A Guide to Commonwealth Government Records.
The Australian War Memorial's online encyclopedia provides a brief summary of events and some useful links to other sources of information.
The Finding Sydney website contains media releases, a roll of honour, and information on the Finding Sydney Foundation.
Wikipedia's article on the search for HMAS Sydney and HSK Kormoran includes information on the discovery of the wrecks and their location, with links to online map and imagery providers.
There are also a number of useful print items available, including:
If you are unable to visit the Library, Trove can help you find and get items at libraries near you.