Parliamentary debates are the full report (largely verbatim) of what was said in the Parliament on official Parliamentary sitting dates. They are commonly referred to as 'Hansard' (in reference to the name of the printing company that first published these volumes).
Use these if you want to see what speeches or statements were made by ministers or other members of parliament in the relevant House about specific issues or events.
Gazettes are the official means of notifying other governments and government agencies, as well as the general public, about government actions and decisions. The Commonwealth Gazette in the past was divided into a number of specialised series (e.g. Public Gazette, Business Gazette). Notices in the gazettes cover local government and electoral matters, appointments and vacancies in the Public Service, and other matters depending on the particular gazette.