The Australian detention centre on Manus Island is described as cruel, inhuman, degrading and violating prohibitions against torture in a detailed report by Amnesty International.
Three Amnesty researchers and translators were allowed into Manus Island for a week last month
Their report, obtained by the ABC, paints a very dark picture of life in the centre for 1100 men detained at three compounds on the island.
Amnesty claims that drinking water in the largest compound, called Oscar, is limited to less than half a litre per day. Water is supplied through 19-litre bottles, the report said.
"They have absolutely no shelter or shade outside and .... the temperature can reach up to 35 degrees. It's extremely humid," said Amnesty researcher Graeme McGregor.
The report also says the men, most of whom tried to come to Australia to escape conflict or poverty in their home country, have to queue in the extreme heat for several hours for meals.
It also claims toilet facilities lack even the most basic services like soap.
"Most of the latrines had no soap when we inspected them ... there are too few showers and toilets to accommodate the number of men in the facility resulting in ... unhygienic conditions," the report said.
Immigration Minister Scott Morrison later told Sky News the government was already acting to address a number of issues raised in the report.
"There are some things in this report that we will seek to verify with Amnesty and get the details of what they're talking about," he said.