The United Nations called on Australia to stop a "humanitarian emergency" unfolding at a detention centre in Papua New Guinea on Thursday, urging an end to a tense days-long stand-off between refugees and authorities.

The Manus Island camp, set up to hold and process asylum-seekers under Australia's harsh immigration policy, was officially closed Tuesday after it was ruled unconstitutional by PNG's Supreme Court.

But some 600 men have locked themselves inside despite water and electricity being cut and dwindling food supplies, saying they are fearful for their safety if they move to transition centres amid reports locals do not want them there.

"UNHCR, the UN Refugee Agency, today reiterates its call to Australia to stop a humanitarian emergency unfolding on Manus Island, Papua New Guinea," the organisation said in a statement.

Describing the situation as "increasingly tense and unstable", the agency said there was inadequate accommodation outside the camp for all of the men, with one centre still incomplete.

Within the camp, they were storing water in garbage bins to sustain themselves, UNHCR said. One Manus detainee, an Iranian called Behrouz Boochani, tweeted Thursday that refugees were "digging into the ground to find water".

"As the days go by where they have no water and no electricity, I think the tensions will just go up higher," UNHCR representative Lam Nai Jit told AFP from Manus Island, adding that the weather was "extremely hot and humid".

He said unease between the refugees and local communities had grown due to a lack of consultation when the transition centres were being constructed.

"The local population have not been prepared... that creates an environment of high risk for both sides," he said.

Lam added that while he had been in contact with PNG authorities, who assured him the detainees would not be forcibly transferred, he had not heard from Australian officials, who had mostly left the area.

- 'Disgraceful' -

The men's plight has attracted the attention of Hollywood, with Australian star Russell Crowe describing the situation as "disgraceful".

He added that he was willing to do his part, tweeting: "I've thought about this. I believe I could house and find jobs for 6. I'm sure there'd be other Australians who would do the same."

The detainees were also terrified of being forced out after control of the camp officially transferred back to PNG's navy Wednesday, advocates said.

But the PNG naval base's commanding officer Begsy Karaki insisted they "will not be forcefully removed".

"There is no panic and those that have pre-conceived ideas developed by the asylum-seekers, I reiterate that there is no threat," he told PNG's Post-Courier newspaper Thursday.

Australia's Immigration Minister Peter Dutton insisted the new facilities were "much better" than being at the camp and said he wanted "to close Manus as quickly as possible".

Australian Greens senator Nick McKim visited the camp this week and on Thursday called on Dutton to end the impasse, saying the refugees were "caught in limbo" between the two countries' governments.

"Each government is trying to claim that the refugees are the responsibility of the other and the men are caught in the middle, and everywhere they turn there is danger," McKim told AFP Thursday.

Barred from moving to Australia, they have been offered transfers to another Pacific on facility on Nauru, but reportedly few have accepted.

A third-country resettlement deal struck between Canberra and Washington has so far only seen 24 refugees flown to the United States.

Resettlement in New Zealand -- raised in 2013 but never taken up by Canberra -- could be viable again after new leader Jacinda Ardern said Thursday she planned to raise the issue with her Australian counterpart Malcolm Turnbull.

The pair are due to meet on Sunday in Sydney.

Canberra has long defended its policy of denying asylum-seekers resettlement in Australia, saying it has prevented deaths at sea.