The main thing that determines whether you are running a business is whether you are going about your activities in a businesslike manner. 


Some of the things that contribute to this are registering a business name, whether you have the intention of making a profit rather than just earning some income, have set up a separate bank account for the business, do you keep accounting records for the activities of the business, and whether you operate from business premises. 


Refer to the Australian Taxation Office website for more info or you may wish to call them on the Personal Tax Info Line 132 861 or the Small Business Info Line: 132 866.