The Greens have hit out at the Australian Government for its "reprehensible conduct" on Manus Island as the stand-off between hundreds of asylum seekers and PNG authorities continues.

On Tuesday hundreds of asylum seekers refused to leave the detention centre, even after power and water were cut and food supplies dwindled, in a deadlock that human rights groups warn could become a humanitarian crisis.

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Greens leader and physician Richard Di Natale has accused the Government of stripping asylum seekers of essential mental health medications.

"Taking away someone's medication — especially in such an abrupt manner — can lead to significant side effects. The conduct is reprehensible," he said.

Senator Di Natale said about 20 per cent of the men had been on medication for mental health disorders.

"Many of the men are clinically depressed, and suffering from post-traumatic stress. They have no further access to their psychotropic medication. It's a calamity, and a humanitarian emergency," Greens Senator Nick McKim added.

Nearly all of the men on Manus Island are suffering from mental health issues, according to a 2015 United Nations report.

Facilities not ready, UNHCR says

Meanwhile Human Services Minister Alan Tudge has rejected claims alternative facilities meant to accommodate the men are still being built.

The United Nations High Commissioner for Refugees Nai Jit Lam is on Manus monitoring the effort to move the men from the detention centre.

Two days ago, Mr Lam inspected two of the accommodation facilities the asylum seekers are supposed to be moved to and says one, called West Lorengau Haus, is not ready.

"There's still major works in progress. We saw heavy machinery still in place," he said.

In a photo from late October, heavy machinery could still be seen at the site of the new accommodation. ( Supplied )

The refugees and asylum seekers say they do not feel safe at the alternative accommodation, but Mr Tudge said such reports should be taken "with a grain of salt".

"From what I understand there has been people from the Manus Island facility who have been travelling to this other destination on a very regular basis," Mr Tudge said.

"Sometimes [they go] to do their shopping, sometimes they have already been staying in that new facility and returning at a later stage."

A Federal Government spokeswoman earlier said the alternative accommodations were "safe and secure".

"[East Lorengau Refugee Transit Centre and West Lorengau Haus] is close to the town of Lorengau, is safe and secure, provides accommodation while [asylum seekers] consider their future," she said.

Buildings at the East Lorengau Refugee Transit Centre and West Lorengau Haus on Manus Island ( Supplied: Federal Government )

'A poorly handled situation': Plibersek

In a final notice posted by immigration authorities on Sunday night, the men were told the PNG Defence Force would start to take control of the site from Wednesday.

It warned the men that anyone choosing to remain would be liable for removal from an active military base.

The asylum seekers, warned that utilities would be cut, had begun to collect rainwater in bins.

However without running water, advocates fear a rapid decline in sanitary conditions of the camp.

Sorry, this video has expired Asylum seekers in Manus Island detention centre say they don't feel safe

The detainees were sharing what food remained but some of the men Reuters spoke to by phone said supplies were running low.

They had been given enough meals to last only until the camp's official closure on Tuesday.

Acting Prime Minister Julie Bishop said the men should move to the new centres, which Australia has said it would support with $250 million worth of food and security for the next 12 months.

Acting Opposition Leader Tanya Plibersek said the situation had been "poorly handled".

"I am worried about what looks like a developing powder keg and I won't pretend I'm not worried," Ms Plibersek said.

ABC/Reuters