Adopted by a Queensland family at the age of six and later granted an Australian passport, Edward McHugh never thought he was anything but Australian.

But the 50-year-old, who was born in the Cook Islands, now faces deportation and being torn apart from his seven children, after the Australian government revoked his passport and told him he was in fact never a citizen.

“I grew up in Toowoomba, Mum and Dad are both Aussies, they never thought they had to do anything for me [to be a citizen] ... I thought I was Australian,” McHugh told SBS News.

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But McHugh discovered that was not the case when he was informed earlier this year he was to be deported on character grounds under section 501 of the Immigration Act due to a criminal conviction.

McHugh was convicted of aggravated assault and making a threat to kill a person in February this year and was sentenced to a nine-month prison sentence.

He believes having served his time he doesn’t deserve to be deported for making "one mistake," he said.

Read more Why New Zealand is furious about Australia’s deportations policy

McHugh is now being held at Brisbane Immigration Transit Accommodation Centre where he is awaiting deportation to New Zealand. All Cook Island nationals have New Zealand citizenship.

He will join the more than 1,000 New Zealanders with criminal records deported from Australia in the last two years after serving time in prison. But Mr Hugh’s case is unusual in that only last year he applied for - and was granted - a passport that clearly states his nationality as “Australian”.

It was the first time he had applied for a passport.

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“It is quite baffling, I was Australian until they wanted to kick me out,” he said.

He is challenging the deportation order.

The Department of Home Affairs told McHugh this year that he had never applied for citizenship by descent and that his adoption did not make him an Australian citizen because it happened before the law was changed.

“Your Queensland birth certificate [granted in retrospect after his adoption] sets out that the date of registration is 4 November 1976. Automatic acquisition visa adoption did not come into effect until 22 November 1984, therefore you did not automatically become an Australian citizen via adoption,” a letter from the department reads.

Read more Acting NZ PM escalates deportation stoush

But the department has so far offered no explanation as to why McHugh was able to use his Queensland birth certificate and adoption papers to successfully apply for and obtain the Australian passport.

McHugh has been on an Absorbed Person visa, according to the department, which it said was revoked after his prison sentence.

“An Australian passport is not evidence of a person’s citizenship,” said the department, which noted passports were issued to non-citizens prior to a law change in 2005.

An Australian passport is not evidence of a person’s citizenship. - Department of Home Affairs

Maree Elliott, Principle Migration Agent at Brisbane based Holt Durham Migration Services, said the case of someone being issued a passport despite not being a citizen was unusual and appeared to be a mistake by the department.

“I’ve only heard of someone being granted a passport by mistake once before, in 25 years of practice,” she said.

McHugh's father Kevin, a farmer in Toowoomba, said he was devastated and shocked to learn his son wasn’t a citizen after all these years.

The family never took steps to secure Australian citizenship because they assumed he had received citizenship by adoption.

“If he goes back to New Zealand where does he go? He hasn’t been there since he was six [years old], he knows nobody,” he told SBS News.

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Rod Hodgson, a partner at Maurice Blackburn lawyers, told SBS News the laws around deportation of non-citizens for criminal offences were "extremely punitive".

"Deporting someone who had lived and worked in Australia their whole life, as an Australian, is punishing them twice," he said.

McHugh says that being deported would be shattering for his family and his kids who are aged between 10 and 28.

“I’ve got to be here for my kids and my grandkids, they are asking for me every day because they are used to seeing my face,” he said.

I’ve got to be here for my kids and my grandkids, they are asking for me every day. - Edward McHugh

A spokesperson for the Department of Home Affairs told SBS News in a statement: “While the Department cannot comment on individual cases, the Australian Government takes seriously its responsibility to protect the community from non-citizens who choose to engage in criminal activity.”

They said questions regarding the issuing of passports should be directed to the Department of Foreign Affairs and Trade (DFAT).

DFAT told SBS News passports are only issued to people it is satisfied are Australian citizens.

"Citizenship laws are complex. On rare occasions, it can happen that DFAT issues a passport on the basis of evidence it has incorrectly assessed as confirming Australian citizenship," a spokesperson said.

"As soon as the Department of Home Affairs confirms that any person is not (or is no longer) an Australian citizen, any Australian passport held by that person is cancelled immediately," the spokesperson added.

Do you know more? Email jarni.blakkarly@sbs.com.au