The United Nations High Commissioner for Refugees is warning of a looming crisis on Manus Island.
The United Nations is warning of a looming humanitarian crisis as Australia's offshore immigration detention centre on Manus Island is shut down.
Australia is working with the Papua New Guinea government to close the centre by October 31.
The federal government says alternative local accommodation and support services have been organised ahead of the centre's closure.
But the UN High Commissioner for Refugees has expressed serious concerns about detainee health, welfare and security.
The refugee agency, which visited the island last month, said local medical services were overstretched and torture and trauma services had been scrapped.
The UNHCR said a lack of planning and consultation, along with the absence of solutions for those not taken under the US resettlement deal, increased an already critical risk of instability and harm.
"Having created the present crisis, to now abandon the same acutely vulnerable human beings would be unconscionable," its regional representative Thomas Albrecht said on Wednesday.
"Legally and morally, Australia cannot walk away from all those it has forcibly transferred to Papua New Guinea and Nauru."
Refugees who have expressed interest in US resettlement may volunteer to transfer to Nauru ahead of the centre's closure.
Asylum seekers who have failed in their bids to seek refuge will soon be thrown out of PNG if they refuse to return to their home countries.
Cabinet minister Michaelia Cash this week twice refused to rule out using force or cutting off water and electricity to coerce people to leave the Manus centre.