A booklet called A Guide to the Wearing of Insignia is available from the Honours Secretariat.
The following summary may be useful, but organisations or hosts often suggest practical amendments to reflect local conditions.
The full insignia is normally worn on a lounge suit or jacket on ex-service occasions such as Anzac Day etc or at Church services. Some ex-service members wear miniatures as the weight of multiple full sized medals might distort clothing.
The miniature insignia is worn to evening functions when 'Black Tie' is specified. If you attend such a function wearing a dark lounge suit, miniatures are still appropriate.
The wearing of the lapel badge is encouraged on civilian clothing at all times. It is not worn with either full-size or miniature insignia.
Members of the Australian Defence Force and other uniformed or ceremonial organisations should follow their dress regulations and customs.
( Thanks to 'Who's Who Guide to Protocol')