Amnesty International inspectors say conditions in an asylum seeker detention camp on Nauru have created a "climate of anguish" for the men being held there.
The human rights organisation, which released its formal report on its findings on Friday, now wants the federal government to close the centre and return the 387 asylum seekers to Australian soil.
Amnesty inspectors spent three days looking around the offshore processing centre and their report describes the conditions as "cruel, inhuman and degrading".
Amnesty said many of the asylum seekers, who are Sri Lankans, thought they had been sent there purely to encourage them to return to their home countries.
But the conditions are "harsh and repressive", with temperatures soaring above 40 degrees being exacerbated by the gravel ground covering and a lack of trees in the compound.
One asylum seeker told the inspectors: "This place is like an oven. An oven for our bodies and an oven for our minds."
Tents were "very cramped", holding up to 16 people with no room to move between the stretcher beds and asylum seekers also complained of insects and rodents.
Inspectors found at least one leak in every tent, which meant bedding and clothing got soaked whenever it rained.
Amnesty refugee expert Graham Thom said officials had prevented Amnesty inspectors from photographing the camp, despite having been told they would be able to take pictures.
He believes the men are being unlawfully detained and called on the government to close the centre and return all the asylum seekers to Australia for onshore processing.
Amnesty said it would be some months before Nauru had the capacity to begin processing asylum claims of the people in the detention centre.
"Why there was any need to send asylum seekers into these conditions when a serious delay in beginning processing was inevitable remains unclear," the report asked.