Australia should immediately release a legally blind and mentally ill Tamil refugee who has languished in immigration detention for nearly a decade, a key United Nations body has said.

In a scathing assessment of the detention regime, the UN working group found Australia had contravened international law by detaining the man for more than nine years. It argued he should be released from detention immediately and had an “enforceable right to compensation and other reparations”.

The Sri Lankan refugee, known as Kumar to protect his identity, was detained after arriving at Christmas Island by boat in March 2010 at the age of 27. He fled Sri Lanka because he had been subjected to persecution and human rights abuses, including torture, amid the country’s decades-long civil war.

Kumar, was found to be a refugee by Australia in December 2010. Yet the 36-year-old has languished in detention, most recently at the Villawood centre. The government initially cited security risks in order to deny Kumar a protection visa, but those assessments were later downgraded by Asio.

“Detaining … a man who is legally blind, has an acquired brain injury and a range of other mental health concerns is an extraordinary act for a developed, democratic nation,” his lawyer, Alison Battisson, told Guardian Australia.

The government argues that Kumar’s detention is legal under Australia’s Migration Act. It has said it is still assessing his most recent application for a temporary protection visa, which was made in October 2015.

But in a legal opinion issued this month, the UN working group on arbitrary detention condemned Australia’s actions, again calling for the government to bring the Migration Act in line with international law.

It found Kumar’s lengthy detention was “arbitrary”, contravened the universal declaration of human rights and the international covenant on civil and political rights. It also said there should be a “full and independent investigation” into his case.

“The working group considers that, taking into account all the circumstances of the case, the appropriate remedy would be to release [Kumar] immediately and accord him an enforceable right to compensation and other reparations, in accordance with international law,” the UN body said.

In 2018, the working group first condemned Australia’s indefinite incarceration of refugees and asylum seekers as arbitrary and illegal.

Kumar’s case is the second this month and the eighth in two years to draw criticism from the UN body. Last year, Guardian Australia revealed the case of Western Sahara man Said Imasi, who at that time had been held without allegation, charge or trial for more than eight years.

The working group has also responded to a ninth case this month, arguing for the release of a Vietnamese woman and her baby detained in Melbourne.

Battisson, the director of pro bono law firm Human Rights For All, said the ordeal has had “devastating consequences” for Kumar, who has been determined as mentally ill by a health tribunal.

He has been periodically moved back and forth from administrative detention to mental health facilities. The government told the UN body he has received treatment for his eye condition, which renders him legally blind.

He also received an acquired brain injury while in Sri Lanka. “His mother emerged safely on the other side and Kumar did not,” Battisson said. “Kumar was found a significant time later in a hospital in a coma. He’s never been the same after that.”

In March 2011 and December 2013, Kumar received adverse security findings, which meant he was not eligible for a protection visa. But in 2016, following an independent review, Asio downgraded that assessment, saying it was “not recommending against the granting of a visa … on security grounds”.

In 2012, Kumar was charged over a fire incident in immigration housing. He was found unfit to plead due to his mental health, and the prosecution dropped proceedings.

Battisson argued any security concerns should have been “totally overridden by the acquired brain injury and the fact … he’s been legally blind for his entire life”.

“Of course for somebody like Kumar, who grew up during the civil war ... in the north of Sri Lanka, there will be contacts and an understanding of the Tamil plight,” she added.

Battisson also questioned why Australia had campaigned to sit on the UN human rights council if it would not “comply with the recommendations of one of its working groups”.

The Department of Home Affairs declined to comment on the case but said it was “aware that the UN working group on arbitrary detention has rendered an opinion”.

“The Department of Home Affairs is reviewing the contents of the opinion, and will respond to the … request for follow-up in due course,” the department said.

“The Australian government takes its international obligations seriously. Australia has a long history of engaging cooperatively with the United Nations and its mandate holders, and welcomes open and constructive engagement with the human rights system.”