A total of six individuals have now ceased to be Australian under changes to the Australian Citizenship Act in 2015 to strip dual nationals of Australian citizenship for actions contrary to their allegiance to Australia.
Under section 35 of the Act, a dual national’s Australian citizenship automatically ceases if they act contrary to their allegiance to Australia by engaging in terrorism-related conduct. This includes those who fight for or who are in the service of a declared terrorist organisation overseas.
Source: Five-foreign-fighters-lose-Australian-citizenship.pdf