Some 600 men were moved out of an Australian-run camp on PNG's Manus Island in November to three transit centers after a local court ruled the facility was unconstitutional.
The men have expressed fears for their safety at the centers, and also accused Australian and PNG authorities of not providing them with adequate healthcare.
In a day-long visit to PNG on Thursday, UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein raised his concerns about the refugees' plight during meetings with the government.
"The government has the responsibility to ensure that while these individuals are on their territory, they have access to their basic necessities and their basic rights, including the right to adequate housing and food," the High Commissioner's spokeswoman Ravina Shamdasani told AFP Sunday.
Shamdasani added that the High Commissioner had also raised such concerns with Australian officials on a number of occasions publicly and in private meetings.
"He will be following up on the cases of these detainees with Australia as well," she said.
The men were brought to Manus as part of a harsh Australian immigration policy that bars asylum-seekers who try to reach the country by boat from resettling there.
Their refugee applications are processed on Manus or at another remote camp on Nauru, before successful applicants are resettled in the Pacific nations or other countries.
But Canberra has struggled to resettle the refugees since the two Pacific sites opened in 2012.
In the past few months, just dozens of refugees from both facilities have been transferred to the US under a deal struck with former president Barack Obama, but criticized by current leader Donald Trump.