Australia has changed its immigration laws to allow for Dual Nationality, meaning that an Australian Citizen can now hold a second nationality and passport. It is recommended that advice is sought on the nationality rules of the other country - not all countries allow dual nationality.
For more specific information about some countries where you hold citizenship:
Under the nationality laws of some countries, a married person automatically has his or her partner's nationality. Children may also have a parent's nationality even if they were born abroad. If your wife, husband or child is visiting the country of your nationality, you should check with the country's consulate or high commission in the United Kingdom before you travel.