SYDNEY (AFP) - Australia's decision to strip a suspected Islamic State in Iraq and Syria (ISIS) fighter of citizenship was thrown into doubt on Wednesday (Jan 2), after it emerged that he was not a dual citizen as once believed.

Neil Prakash was accused of being a member of the militant group, and was announced late last month as the 12th Australian dual-national to lose their passport over terrorism links. He was reported as having both Australian and Fijian citizenship.

But the authorities in Fiji told local media that Prakash was not in fact a Fijian as Australian authorities believed, prompting questions about the legality of the Australian government's move.

Australian law allows citizenship to be revoked if that person is a dual citizen.

Prakash - a suspected senior recruiter for ISIS who has been linked to terror plots to kill Australians - is currently facing charges in Turkey of joining the organisation.

Fiji's Immigration Department director, Mr Nemani Vuniwaqa, told the Fiji Sun: "Neil Prakash has not been or is a Fijian citizen. He was born in Australia and has acquired Australian Citizenship since birth."

He is thought to be eligible for Fijian citizenship through his parents, but "he has not entered the country nor applied for citizenship", Mr Vuniwaqa was quoted as saying.

The revelation was embarrassing for Australian home affairs minister Peter Dutton, who was accused of incompetence by the opposition Labor Party.

In a statement, Mr Dutton indicated that the decision to revoke Prakash's citizenship still stood and was taken in consultation with "several government departments".

"The Government has been in close contact with the Government of Fiji since Mr Prakash was determined to have lost his citizenship," Mr Dutton said. "Australia will continue our close cooperation with Fiji on this issue and the many other areas of mutual interest."

But the decision now appears certain to face further legal scrutiny, with debate centring on whether eligibility to hold another passport is enough to revoke Australian citizenship.

Australia is also a party to a decades-old United Nations convention on stateless persons.