Encylopaedias are a great place to start your research - they provide succinct commentary on your topic and list leading and illustrative cases.
Halsbury's Laws of Australia is arranged by topic and includes commentary and leading and illustrative cases.
The Laws of Australia is another source of encyclopaedic commentary which can be browsed or searched and is arranged by topic and includes commentary and leading and illustrative cases. To browse, scroll to Topic 19: Government in the alphabetical topic list and then use the + sign to keep opening up to more and more specific topics. All topic 19 sub-topics are on constitutional law, beginning with Constitutional Law at 19.1.
Journal articles provide current scholarly opinion and analysis of specific aspects of your topic. They are more in-depth and often more up to date than books or encyclopaedic commentary.
Don't restrict yourself to searching Australian journal databases - many others contain articles about Australian law. We recommend:
Constitutional Law & Policy Review (ceased publication with vol 13(3) in 2011).
Public Law Review