The UN refugee agency on Friday criticised conditions at a detention centre in Papua New Guinea where Australia sends asylum-seekers, wading into an issue looming large in Australia's forthcoming election.

A recent UNHCR visit underlined major concerns over the Manus Island centre, said spokesman Adrian Edwards.

"Our inspection revealed continued and worrying shortcomings. Freedom of movement is still extremely limited in what continues to amount to an environment of open-ended, mandatory and, in UNHCR's view, arbitrary detention," he told reporters.

"The combination of a tough physical environment, restricted legal regime, and slow processing mean that existing arrangements still do not meet the required international protection standards," he added.

Edwards said there had been improvements since a January visit, including the transfer of detained women and children to Australia, and that staff were working hard in "very challenging circumstances" to help detainees.

"But current arrangements still do not meet international protection standards for the reception and treatment of asylum-seekers," he said.

Most of the asylum-seekers are from Vietnam, Pakistan and Iran, he noted.

Canberra has attempted to beat people-smuggling by sending those arriving by boat on its remote offshore territories to processing stations in Papua New Guinea and the tiny Pacific nation of Nauru.

The UNHCR has repeatedly slammed the policy, saying that while Australia has a generous official refugee programme, there has been a widening range of deterrent measures proposed or in place to try to stop boat-people.

Newly reinstated Prime Minister Kevin Rudd last week backed talks with countries of origin to try to stop boats making the perilous journey to Australia, during which many die, after paying huge fees to smugglers.

Rudd has poured scorn on conservative opposition leader Tony Abbott's plan to "turn back" the boats, saying this risks a diplomatic flare-up with Indonesia, a major transit point.

In his previous stint as premier up to 2010, Rudd relaxed tough refugee controls. But he is now under pressure to take a hard line.

His predecessor Julia Gillard, tipped to suffer a crushing defeat at Abbot's hands in September polls, was ousted last month in a Labor party coup.