Amnesty International's Dr Anna Neistat says the conditions faced by refugees on Nauru are some of the worst she's seen during her career.
These comments come following by Amnesty and Human Rights Watch which found that detainees were routinely neglected, sometimes denied medical care and subjected to abuse and violence from Nauruans.
The undercover investigation saw representatives from both groups spend 12 days on the island last month.
Amnesty International said it had unsuccessfully tried several times to get permission to visit the country, but Dr Neistat managed to travel their legally to see the conditions for herself.
"Quite honestly, I can tell you that that I've been coming to war zones and closed countries for the last 15 years and the situation on Nauru stands out, it does stand out because of almost senseless cruelty and injustice that is happening there," she said.
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She says while the detention centre is an open facility, Nauru is tiny.
Dr Neistat also accused Australia of breaching international law by subjecting people to indefinite detention.
"The island is 21 square kilometres so it's smaller than some of the prisons in the US, for example," she said.
"So I think the comparison is valid ... this indefinite detention of refugees and asylum seekers is also totally against international law. The fact that people have no safety and are being deprived of their most essential human rights also speaks to the fact that Australia is in breach of its international obligations."
Michael Bochenek of Human Rights Watch told the ABC that all the detainees complained of mistreatment by the Nauruans.
"I had people tell us that every time they ventured out into the community, they are at least verbally harassed, sworn at, cursed, spit upon, have bottles thrown at them and other objects," he said.
"People have described being swerved at as they walked down the side of the road and we have much more serious physical attacks. People attacking with pipes, with sticks, drawing blood, and when refugees and asylum seekers complain to police about it about the actions of people in the local community, those complaints are very often met with no response whatsoever."
Amnesty released video footage of the interviews it conducted with some of the detainees.
A female detainee said she totally misjudged Australia.
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"We chose Australia because at least we though it's a first-world country and it's supposed (recognition of) women and children rights [sic], but I think we were just wrong. I mean they're tortured [sic] us for three years now," she told Amnesty.
She said being on Nauru has almost robbed her of the will to live.
"I would much rather die in the ocean than stay here, and [I] die daily by seeing my mum breaking down and see how she tried to hold up together [sic]. I mean it's hard."
A spokesman for the Department of Immigration and Border Protection said there was no consultation between the Department and Amnesty International in preparation of this report.
The Department strongly refutes many of the allegations in the report.
