Colin Martin admits he is frightened because by the end of the month he will be on a plane to Canada, the home of his birth — but a place he knows little about.

"To be honest with you I'm scared because I don't know anything," the 26-year-old told the ABC over the phone from a detention facility.

"You know it's another country — I've grown up in Australia — I'm Aussie."

He came to Australia as a toddler when his Australian-born mother and Canadian father moved to Australia in 1994.

"I've grown up here, I've done all my school here," he said.

But school was difficult and he was diagnosed with an intellectual disability when he was 14, around the same time he started offending and taking drugs.

Mr Martin says the only home he knows is Australia. ( Supplied )

Deported under 'bad character' test

It has been almost a year since he was picked up by Immigration officers at Dandenong Magistrates Court in Melbourne's east, and told he was an overstayer.

He's also been deemed of "bad character" by the Home Affairs Minister Peter Dutton because of a long history of criminal offending.

Timeline: 1992: Colin Martin born in Canada

Colin Martin born in Canada 1994: Moved to Australia

Moved to Australia 2006: Diagnosed with a learning disability, aged 14

Diagnosed with a learning disability, aged 14 2007: Started using heroin and marijuana

Started using heroin and marijuana 2011: Convicted of taking part in armed robbery, sentenced to 2 years' jail

Convicted of taking part in armed robbery, sentenced to 2 years' jail 2017: Detained by immigration officials to be deported

Detained by immigration officials to be deported May 2018: Appeal against deportation dismissed

"I didn't know in doing these offences that I'd be thrown out of the country away from my friends and family," he told an appeal hearing of the Administrative Appeals Tribunal (AAT) last week.

"How can I be unlawful at three years old," he asked AAT senior member Andrew Nikolic who was presiding over the matter.

The AAT heard Martin had committed more than 150 offences — mostly graffiti and other property offences, but also drugs and driving or traffic convictions.

The most serious crime was taking part in an armed robbery of a Subway franchise in November 2011.

He served two years in a youth centre.

On Wednesday the Administrative Appeals Tribunal slammed the door on any slim hope he had that he would be able to stay.

He was advised that the AAT was upholding a decision in February to refuse him a bridging visa based on the "serious and repetitive nature of his offences over an approximately eight-year period".

Mr Nikolic set out his reasons in a 47-paged report obtained by the ABC, in part because "the … risk of continuing to engage in substance abuse and committing further offences remain very real".

"The evidence supports a finding that if allowed to remain in Australia, Mr Martin constitutes an unacceptable risk to the Australian community," Mr Nikolic said.

He concluded that "Mr Martin does not pass the character test".

Neil Martin said his son has been left "depressed" and "deflated" by the deportation proceedings. ( ABC News: Barry Pullen )

No 'human decency'

To get to this point has been a long, hard ride according to Colin's father, Neil Martin.

"He's not the boy he was 12 months ago," he told the ABC.

Mr Martin said his son was "depressed … he's deflated".

At first he was put in the Maribyrnong Detention Centre, in Melbourne's north-west, where he alleged he was sexually assaulted by guards, something the Australian Border Force refutes.

"Mr Martin's claims have been thoroughly investigated by the ABF and our service provider and no evidence has been found to substantiate the claims," it said in a statement.

Neil Martin said the department had failed to treat his son with "common courtesy or human decency".

Since June 2017, Colin Martin has been flown back and forth between Maribyrnong and the Yongah Hill detention centre, near Perth, six times.

This graph shows the number of Australian visas cancelled (blue) and refused (red) on character grounds since 2010. ( Supplied: Department of Home Affairs )

System 'designed to mentally break you'

It's a common experience for the "501-ers" — as they're known inside the detention centres — named for those affected by section 501 of the Migration Act which refers to bad character.

Brisbane immigration agent Richard Timpson said "it's a regular occurrence" that frustrates him and other immigration lawyers who are trying to manage cases with clients in another part of the country.

He said "pressure can sometimes be exerted" on detainees to sign in those circumstances, because sometimes "it's easier to leave and protest the matter offshore".

Department of Home Affairs statistics show the number of visas cancelled on bad character grounds has more than doubled from 2014-15 to 2016-17, and increased 15-fold since 2013-14.

"Minister Dutton keeps on saying he wants to rid the great country of Australia of violent criminals, that my son is not, and nor has he ever been," Neil Martin said.

Colin Martin said he was pressured up to 20 times to sign a voluntary removal form which would expedite his deportation.

Even before the AAT's final ruling, Colin Martin broke down and signed three weeks ago.

"This whole system is designed to mentally break you, so you sign to go back, that's why they fly you away from your family," he said.

Mr Martin has been told he will be deported on May 29.