Kevin McHugh is holding his son's birth certificate. He doesn't understand why it appears to count for nothing now.

Photo: RNZ / Supplied

"I wish someone could tell me [what's going on], I can't understand how you can adopt someone and find out they're not an Australian citizen," says the retired Towoomba dairy farmer.

Kevin and his wife adopted Eddie McHugh when he was eight, after he arrived aged six from the Cook Islands, where he was born in 1968.

Eddie's passport, which he was issued last year, says: "Nationality: Australian."

But the 50-year-old Broome artist and one-time machine operator has been in immigration detention in Sydney since May, threatened with deportation to New Zealand over a 2005 conviction for possessing drugs and a four-month prison sentence this year for assault.

"He's got a minor criminal record as an Australian, not as a Kiwi," his father said.

"He's had a little bit of trouble, strife with the law, and they've just said, well, let's just send him back there.

"I can't find where the mistake is and no one wants to tell us."

Documents show Australia's Home Affairs Department thinks there is no mistake.

These say that Eddie McHugh is not a citizen, got his passport by mistake and must be detained on character grounds because of his 16-month prison sentence on drugs in 2005.

"Therefore, the Australian passport issued to you on 25 October 2017 was done so in error."

As for his adoption it was only after a law change in 1984 that a child who was adopted automatically got citizenship.

If Mr McHugh's appeal fails, he either remains detained or agrees to deportation, and leaving his seven part-aboriginal children behind.

"No, I don't accept that. They say I'm not adopted - they didn't know I was adopted until I told them. Before I told them that, they put me on an Absorbed Person Visa. I didn't have a visa in the first place."

By putting him on a visa, the department was then able to cancel the visa and detain him when his release date from prison in May fell due.

This was done on character grounds under the notorious 501 laws: he failed this test due to a conviction for possessing drugs with intent, for which he got a 16-month prison sentence in 2005, and a four-month sentence for assault in February this year.

"I didn't touch anyone. I was yelling at him," Eddie McHugh told RNZ from detention at Villawood. "Nothing that I've done ever is a reason to deport me."

A case far from unique

He has been shifted from Perth, to Christmas Island, to Brisbane and now to Sydney, thousands of kilometres from any family who could visit.

He is still waiting for an appeal to be heard, is trying - and failing - with freedom of information requests, but remains upbeat, believing because he has an Aboriginal family the state cannot deport him.

Kevin McHugh is less certain, and the uncertainty is taking its toll.

"I've had a stroke. Still going to do tests. Three months ago I didn't even know what a Panadol was, to taking seven tablets a day now."

Eddie McHugh did not apply for citizenship by descent, so has no claim, the Department of Home Affairs said. He said he never knew he should apply.

The case was far from unique, said an advocate for detainees Filipa Payne of Christchurch.

"There's at least four other people I know in detention who have been adopted. They've thought that they were Australian citizens throughout their entire life," Ms Payne said.

She knew of three other adopted men who have already been deported since immigration laws were tightened four years ago.

The New Zealand Government has asked Australia to only deport people with a genuine affiliation or roots here.