New Zealand PM denies deal over Australian asylum-seekers

Prime Minister John Key flatly denies New Zealand is about to sign a deal with Australia to resettle refugees detained on Manus Island and Nauru.

Speculation about such a deal has been prompted by the Australian Prime Minister Malcolm Turnbull's announcement that asylum-seekers who arrive by boat will be prevented by law from ever being let into the country.

The denial comes as a United Nations investigator arrives in Australia today for an 18-day visit to immigration detention centres.

The proposed Australian law would cover asylum-seekers on Manus Island and Nauru.

New Zealand has offered to take 150 people from Nauru a year but Mr Key said while the offer remained on the table, Australia showed no inclination to take it up.

However, a source told the ABC the only way the proposed new law would make sense was if the Turnbull government was close to signing a deal with another country, like New Zealand.

Mr Key told reporters that was not happening.

"No - it's not New Zealand [that has been approached]."

Labour leader Andrew Little said the proposed legislation was drastic.

New Zealand would never be able to resolve the issue for Australia, but should continue to offer to assist in whatever ways were helpful, he said.

Meanwhile, United Nations special rapporteur on the human rights of migrants Francois Crepeau arrives in Australia today.

Mr Crepeau will use his trip to examine immigration detention and processing centres in Australia and on Nauru.

In a statement, Mr Crepeau said he would also meet with border protection officials and migrants as part of a series of meetings in capital cities.

"This is an opportunity for me to understand how Australia manages its overall migration policies, and their impact on the human rights of migrants," he said.

The visit comes 14 months after Mr Crepeau cancelled a planned trip due to concerns over immigration legislation.

At the time, he issued a statement regarding the Border Force Act, voicing concerns that the legislation prevented him from "fully and freely" carrying out his duties.

"This threat of reprisals with persons who would want to cooperate with me on the occasion of this official visit is unacceptable," he said.

"The act prevents me from fully and freely carrying out my duties during the visit, as required by the UN guidelines for independent experts carrying out their country visits."

Mr Crepeau's visit coincides with the Australian government's announcement on proposed amendments to the Migration Act, which would ban refugees and asylum seekers on Manus Island and Nauru from ever coming to Australia.