Australia must show “political courage” and end the indefinite detention of asylum seekers held on Nauru, the new Pacific representative of the United Nation human rights office of the high commissioner has said.

In an interview with Guardian Australia Chitralekha Massey, spoke at length about Australia’s offshore detention policies.

She said Australia’s detention of asylum seekers on Nauru “is unsustainable, it’s a violation and it’s unnecessary”. The continuing reports of abuse, self-harm and sexual assault had created an alarming environment at the centre that Massey said needed be resolved.

The Guardian’s publication of the Nauru files has put the treatment and conditions of asylum seekers held on the Pacific island back into the spotlight.

Massey said that the asylum seekers held on Nauru had to be given the opportunity to resettle in Australia or another country if they had been found to be refugees.

United Nations general assembly and a key migration summit hosted by the United States president, Barack Obama, will be held next week. The Australian prime minister, Malcolm Turnbull, and the immigration minister, Peter Dutton, will be both be in attendance.

Massey encouraged the Australian government to bring an end to the detention of men, woman and children on Nauru, where hundreds of asylum seekers languished in “not only prolonged, but indefinite” detention.

Massey’s team completed an inspection of the Nauru centre in August 2016. She said the situation in the past two years had been “aggravated even further”, based on the observations of her office’s latest visit.

“We have repeatedly reported our concerns around healthcare, around education, access to justice,” she said.

“We are particularly concerned about the sexual violence and the number of rapes against women, against children. The amount of self-harm that is really an indicator of how bad the mental health of people has deteriorated.”

“It’s difficult for me to find words to describe how somebody can go from being at a breaking point and just continue to be at a breaking point endlessly.

“There is an urgent response required.”

In response to Dutton’s comments that Australia’s regional processing relationship with Nauru would continue for “decades” she said: “It’s already unacceptable.”

She urged Australia to adopt a “non-custodial” approach to migration and those seeking asylum: “There is a better way to do this.”

“It’s very important that the children on Nauru receive, with urgency, the chance for normal lives.”

She said she looked favourably at the Australian Human Rights Commission’s recent report Pathways to Protection, which canvassed a number of options to encourage a more humane and regional response from Australia.

She also noted that while the detention centre on Nauru moved to an “open centre” arrangement in October 2015, asylum seekers held on the island still lived in “militarised, police-like” arrangements. She also urged Australia to take up a greater regional role in bolstering the human rights framework of neighbouring Pacific countries.

The UN office continues to focus on other aspects of Australia’s approach to human rights. Massey said the office remained concerned about the juvenile justice system, Indigenous affairs and gender-based violence in the community.

“We are very concerned that juvenile justice seems to be almost the norm rather than the exception,” she said.

A royal commission is due to to begin shortly into the Northern Territory’s juvenile justice system.