Professor Ergas said Australia once demanded that citizens didn't merely vote, but also actively engaged and showed signs of loyalty and respect, including demanding willingness to make "the ultimate sacrifice" through conscription. With the disappearance of compulsory military service, the sense of commitment had declined, a "serious concern", he said.

Experts were split on last week's landmark decision at the hearing on Friday.

Professor Ergas said the High Court decision was contrary to the move away from "any biological or racial basis" for membership of the community.

"The reason we allow for considerations such as indigeneity to play a role is not so as to cement it in perpetuity. It is not to entrench differences. It is to close gaps and move towards genuine political equality," he said. The High Court decision did the exact opposite.

But Professor Helen Irving from the University of Sydney said Professor Ergas's idea of citizenship was "extremely masculine". Women were not conscripted and until 1948 they lost citizenship automatically if they married a foreign man, and it was not only citizens who were conscripted, she said.

The government was forced to release convicted criminal Brendan Thoms from detention last week instead of deporting him to New Zealand after the High Court ruled that Aboriginal people with traditional connections to their communities could not be aliens. The court was split 4-3 in the decision, which created a group of people who are neither citizens nor aliens.

University of Canberra constitutional expert Kim Rubenstein said the High Court had affirmed the original status of Indigenous Australians at the time of federation and set "some boundaries beyond which the executive cannot pass".

She criticised the government's move to strip dual citizens of Australian citizenship when they have committed offences, saying it made people with dual citizenship different and more vulnerable than people with only one citizenship.

Australians were also being removed to countries with which they had no association other than a citizenship certificate, a move that hailed back to Australia's days as a penal colony, she said.

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Liberal Senator Amanda Stoker said there was a compelling argument that the High Court's decision was harmful to unity.

But Prof Rubenstein said it was not the first time the High Court had identified non-citizen non-aliens. British people who had lived in Australia all their lives without citizenship were protected for a brief period, and even today some British citizens were able to vote in Australia despite not having citizenship.

Professor Helen Irving from the University of Sydney said the High Court case put the validity of the government's attempts to prevent dual citizens, including foreign fighters, from returning.

Citizenship was a neutral international status and values or character should not be relevant, she said.

Senator Stoker asked whether anything could justify stripping someone of their citizenship to which Professor Rubenstein said the only case would be where someone worked actively to end the nation - such as if someone fought with a foreign country set on destroying Australia.