Australian Immigration Minister Peter Dutton confirmed Tuesday the centre will start shutting down in parts in the coming months ahead of its final closure in October.
"The centre won't close drop dead on the 31st of October, they will start to decommission parts of the centre in the run-up," Immigration Minister Peter Dutton told 3AW radio in Melbourne.
Papua New Guinea officials reportedly told Manus refugees on Monday that an area of the camp on the secluded island would close next week, while the rest of the compound will shut on June 30.
"You cannot stay at the regional processing centre. You need to consider your options. No one will be resettled in Australia," one official was quoted as saying.
The detainees, who were sent there by the Australian government after they tried to reach Australia by boat, were also told that they would be moved to a transit camp, according to local media.
David Manne, the executive director of Refugee Legal, said detainees continue to be trapped, calling the limbo dangerous and destructive.
Greens Party parliamentarian Nick McKim said he had grave fears for hundreds of men and the difficult choices the face.
"They could ... go back to the countries they have fled from and face persecution, personal danger, arbitrary imprisonment or worse," Senator McKim told reporters in Hobart on Tuesday.
"They were told they could go to a third country such as Cambodia - which we know doesn't have necessary support for refugees - or they were told they could take their chances in Papua New Guinea."
He said he feared there is a possibility some detainees may die as a result of the announcement.
More than 800 people are currently languishing in the centre, which was deemed illegal by Papua New Guinea's Supreme Court last year, and a majority of them have been found to be genuine refugees.
The United Nations, rights groups and even an Australian parliament inquiry criticized the country's immigration detention policy as well as the deplorable living conditions in the centres on Manus and the Pacific island of Nauru.
Behrouz Boochani, a Kurdish-Iranian who has been in detention on Manus island for four years, said on his Twitter: "Manus prison is full of tension."
"The refugees are very angry with immigration's plan to settle them in PNG," he posted, and later told broadcaster ABC that many detainees would resist and there could be a riot if officials use force.
Most of the detainees are waiting to be accepted by the United States in a refugee resettlement deal with Australia, but it is not certain how many will be taken and when it will take place.
Dutton said Tuesday that refugees who are not taken under the deal may settle in Papua New Guinea, while roughly 400 people deemed not to be refugees will be sent back to their home country.
"I hope they now realize that the government is deadly serious about the fact people will not be coming to this country under any circumstance," he told Sky News on Tuesday.
It was not an option for refugees to relocate to a transit centre, nor resettle in the Papua New Guinea community since others before them have found it to be too hard or dangerous, said John Falzon from the St Vincent de Paul Society.
"Nor was it an option for detainees to return to countries of origin when it could result in persecution or harm," he said.
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