It is possible to hold citizenship of two or more countries if the law of those countries allow. This is known as dual, or multiple, citizenship.
People can become dual citizens automatically, or after being granted citizenship of another country.
For example, an Australian citizen may automatically gain citizenship of another country through marriage, while a permanent resident of Australia may become a dual citizen by becoming an Australian citizen.
Prior to 4 April 2002, Australian citizens who became citizens of another country lost their Australian citizenship automatically.