Canberra has announced a "one- off" deal with Washington which will see refugees in Australia's offshore detention centres resettled in the United States.

Prime Minister Malcolm Turnbull said the deal would apply only to refugees currently being held on Nauru and Manus Island, Papua New Guinea. He would not specify how many of the more than 1,200 detainees would go to the US.

Warning that the deal will not apply to future asylum seekers sent to the island centres, he said yesterday: "It is a one-off agreement. It will not be repeated."

The priority is the resettlement of women, children and families, he told reporters.

"This will be an orderly process. It will take time. It will not be rushed," he added.

Confirming the deal yesterday during a visit to New Zealand, US Secretary of State John Kerry encouraged all nations to cooperate in resettling the world's refugees.

"We in the United States have agreed to consider referrals from (the United Nations High Commissioner for Refugees) on refugees now residing in Nauru and in Papua New Guinea," he told reporters.

"We will remain focused on trying to save lives and on providing timely humanitarian assistance and ensuring that the human rights of all migrants are respected."

The deal was largely welcomed as a breakthrough that could end a sorry saga for Australia, whose controversial offshore detention centres have long been described as illegal and cruel by the United Nations and human rights groups.

US Homeland Security officials are expected to arrive in Australia within the next few days to begin vetting potential refugees for resettlement. Those who do not want to go to the US will be offered 20-year visas for Nauru.

Australia has one of the world's toughest policies on asylum seekers, including towing back boats to Indonesia and transferring arrivals to Nauru or Manus Island.

The move has effectively ended the flow of asylum seekers but left many languishing in secretive centres on the remote islands for years.

Mr Turnbull did not say whether he had raised the deal with US President-elect Donald Trump during their conversation last week.

Mr Trump has proposed banning Muslims from the US, and many of the refugees in the Australian centres are Muslims from countries such as Iran, Iraq and Afghanistan.

The deal was cautiously welcomed by the UN agency, which agreed to refer refugees for potential resettlement, but stipulated that Australia's international legal obligation was to allow people to seek asylum.

The agency said it "remains gravely concerned" about vulnerable detainees, adding that "appropriate solutions" must be found for all.

Human rights groups delivered a mixed response, with some warning that Australia was shirking its international responsibilities.

"Australia is one of the wealthiest countries in the world and should be leading by example at a time of global record high levels of people forced to seek safety," said Amnesty International in a statement.

But others welcomed the long- awaited closure of the controversial offshore centres where there have been multiple alleged rapes and assaults, attempted suicides and riots.

Some noted the deal has no specific time frame for when the refugees will be transferred.

"The pain and misery that have been felt by asylum-seeker children and their families on Nauru and Manus Island have been a black mark on Australia's conscience," said Mr Paul Ronalds, chief executive of Save the Children Australia.

"We must celebrate that these children are now finally being given the opportunity to fulfil their full potential."