Amnesty International joined 80 other NGOs on Thursday in urging Pacific leaders demand the closure of an Australian-funded immigration detention camp on Nauru when they meet in the tiny island nation next week.
The 18-nation Pacific Islands Forum (PIF) will hold its annual summit in Nauru from September 3-6, with delegates meeting just a few kilometres from the camp dubbed "Australia's Guantanamo".
It houses asylum-seekers who have tried to reach Australia by boat and are processed in remote facilities on Nauru and Papua New Guinea under Canberra's hardline immigration policies.
Australia says offshore processing is necessary to deter illegal boat arrivals and prevent deaths at sea on the treacherous journey.
Amnesty, along with the 80 other non-government organisations, released an open letter calling on PIF leaders to act and end "a stain on the region".
"Pacific island leaders cannot ignore this issue any longer and need to ensure that it is at the very top of the forum's agenda," Amnesty's Pacific researcher Roshika Deo said.
"This is a desperate situation that requires urgent action. Regional leaders must show that they will not stand by while the Australian government's abusive policies continue to risk more lives."
The rights groups said asylum-seekers on Nauru and PNG's Manus Island were subjected to "cruel and degrading treatment" that must stop.
"(There are) widespread reports of violence against refugees in Papua New Guinea and violence and sexual harassment of women and children on Nauru," the letter said.
There are currently more than 200 people in the Nauru facility, according to the Refugee Council of Australia, including dozens of children.
Rights advocates say they are suffering mental health issues under the strain of indefinite detention, with reports of despondent children harming themselves.
However, the Canberra-bankrolled facility has been an economic lifeline for Nauru, which has an area of only 21 square kilometres (eight square miles) and has depleted its only natural resource, phosphate.
The Nauru government has imposed strict conditions on media covering the PIF summit, threatening to revoke journalists' visas if they capture images of the camps or asylum-seekers.
It has also limited the number of reporters attending and barred Australia's public broadcaster, the ABC, after taking exception to its coverage.