A young Iranian refugee who was held on Manus Island for more than three years has fled to Fiji, where he is seeking asylum on the grounds he fears persecution if he goes back to Papua New Guinea.

"This is the end for me," said Loghman Sawari, whose time in PNG has been punctuated by beatings, bullying, imprisonment, illness, suicide attempts and living on the street in Lae, one of the country's most dangerous cities.

The 21-year-old Ahwazi Arab managed to board a plane under a false name after he said he was threatened by a PNG immigration official and lost hope of being resettled in the United States under President Donald Trump.

It is the first case where a refugee who sought asylum from Australia is claiming to be fleeing persecution by a country connected to the Coalition's deliberately harsh "stop the boats" policy.

Mr Sawari told Fairfax Media he had cobbled together the money for the airfare from several sources over several months.

Broke and destitute, he has been given refuge by a Fijian family and is planning to present himself to Fiji immigration officials on Monday to seek protection.

UNHCR officials are understood to be aware of his situation, which poses legal and diplomatic challenges for the countries involved. Fiji, PNG and Australia are all signatories to the refugee convention.

A warning that Mr Sawari could take extreme measures was included in a submission from trauma worker Janet Galbraith 10 weeks ago. The submission requested he be included in the proposed US resettlement deal because of his vulnerability.

"Hopelessness and living in constant fear is leading him to consider drastic action. I also believe that his mental health is declining and is at a point where he needs to be given the support and safety very soon," Ms Galbraith wrote to the UNHCR.

"I am very concerned for him. He is exhausted and I believe he is unable to continue. I also believe he will make some decisions that will see him either lose his life or at least end up more damaged."

Mr Sawari said he is terrified of being sent back to PNG. "If I go back they will make me crazy or they put me in the jail. I'm sure about that," he said by phone from Nadi.

"My problem is I cannot go back to Iran and I can't stay in PNG. I don't want to go to Australia. I just want to be free, just like human being. I would stay in Fiji. Here you can walk anytime - night, morning. You free here."

Ms Galbraith has just returned from three months in PNG and said Mr Sawari is more vulnerable than many others on Manus Island because his teenage years were filled with trauma and torture in Iran before his ordeal on Manus Island.

For the first few months of his detention in Papua New Guinea, Mr Sawari was an aberration: the boy in a detention centre that was supposed to be exclusively for men, a number of whom have wives and children in Australia.

He was 17 when he arrived on Manus Island in August 2013, one month after the then Labor government decided to remove children and family groups from the detention centre. A letter from Australian immigration officials told him he would be "treated as a minor for the purposes of accommodation, placement and other purposes". He remained in isolation until his 18th birthday, when he was told he would be staying.

He was one of the first refugees to be leave the detention centre on Manus Island after his claim for refugee status was accepted, but he suffered from depression and was taken to the local jail after attempting suicide soon after his release. Case notes dated January 2014 show that Mr Sawari was considered to be "high risk".

He lasted two months in PNG's second-biggest city, where he said he was terrified by an armed "raskol" and reduced to tears by bullying in the town before being befriended by a homeless youth.

After being refused permission to return to the transit centre, Mr Sawari fled on a boat and was protected by locals on a neighbouring island for some weeks before returning to Manus Island and being transferred to Port Moresby.

In Port Moresby, Ms Galbraith's report notes Mr Sawari continued to experience harassment, bullying and attacks. "He is extremely and obviously hyper-vigilant all the time. He seldom sleeps and often spends a week inside his unit as he is afraid to venture out."

Ms Galbraith said she had no warning of Mr Sawari's intentions, but had counselled him against previous escape plans, which included travelling by boat to the Solomon Islands.

"In PNG, many of the refugees are talking about doing this. He's just the first who has succeeded. It's not that others haven't tried."

He has also suffered from chronic dental problems, recurring infections in his ears, gums, throat and stomach since 2013 and bouts of fainting and dizziness.

His voice sounded excited and hopeful when he spoke to Fairfax Media soon after arriving in Fiji.

But the excitement had given way to anxiety, uncertainty and dread on Friday. "Today I'm not really OK because I'm thinking too much what will happen."

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