About 50 asylum seekers have departed an Australian-run detention camp in Papua New Guinea after police moved into the complex, confiscating food, water and personal belongings from the roughly 310 who remain.

The Manus Island centre was sealed off after a three-week stand-off the United Nations has called a "looming humanitarian crisis" as detainees defied attempts by Australia and Papua New Guinea to close it.

"Right now we have no water," one of the asylum seekers in the camp said in a mobile telephone message. "I came back to my room and they took my laptop and money and cigarettes."

Video images shot and posted on social media showed police using a megaphone to tell asylum seekers to leave because their stay at the camp, located on land used by the Papua New Guinea navy, was illegal.

Men boarded buses in footage posted on Twitter. It is understood that the buses took the men to alternative accommodation.

A police blockade of the camp was still in place, Tim Costello, chief advocate of aid group World Vision Australia, said by telephone from outside, adding that he had seen buses leave and that the accommodation he had visited was unfinished.

Papua New Guinea immigration and police officials have not made any comment.

The camp, and another on the tiny Pacific island nation of Nauru, have been the cornerstones of Australia’s controversial immigration policy, which has been strongly criticised by the United Nations and rights groups.

Australia opened the camps in a bid to stem a flow of asylum seekers making dangerous voyages by boat to its shores.

Under its "sovereign borders" immigration policy, Australia refuses to land asylum seekers arriving by sea, and sends them to the offshore camps instead.