The United Nations special rapporteur on the human rights of migrants has postponed a planned trip to Australia, claiming the Federal Government's immigration legislation preventing people talking about what happens in detention facilities does not allow him to carry out his job.

Francois Crepeau said he was invited to travel to Australia by the Commonwealth and planned to gather information about the situation of migrants and asylum seekers in the country and on offshore detention centres.

He was due to arrive on Sunday.

"In preparing for my visit, it came to my attention that the 2015 Border Force Act, which sanctions detention centre service providers who disclose 'protected information' with a two-year court sentence, would have an impact on my visit as it serves to discourage people from fully disclosing information relevant to my mandate," Mr Crepeau said in a statement.

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

Francois Crepeau says the Border Force Act would prevent him from freely carrying out his duties. ( AFP: Karim Jaafar/Al-Watan Doha )

"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 said he had asked the Government to give him a written guarantee people he interviewed during his tour would not be at risk of sanctions under the Border Force Act.

"As the Australian Government was not prepared to give the written assurances required by the official terms of reference for fact-finding missions by special rapporteurs, it was not possible for me to carry out the visit in my capacity as a UN independent expert," he said.

"Since March 2015, I have repeatedly requested that the Australian Government facilitate my access to its off-shore processing centres.

"I was also extremely disappointed that I was unable to secure the cooperation needed to visit any offshore centre, given the international human rights and humanitarian law concerns regarding them, plus the Australian Senate inquiries on the offshore detention centres in Nauru and Papua New Guinea, which raised concerns and recommendations concerning these centres."

Australia ready to arrange UN's future visit

A spokesman for Immigration Minister Peter Dutton said Mr Crepeau's decision not postpone his visit was "disappointing".

"The Government accommodated to the fullest extent possible the requests of the office of the special rapporteur as it has with past visits," the spokesman said in a statement.

"The Department of Immigration worked closely with the office of the special rapporteur to develop a comprehensive program and facilitate engagement across government.

"The program included meetings with key government officials, serviced providers, other organisations and visits to the detention network."

The spokesman said access to offshore processing centres in Papua New Guinea and Nauru was the responsibility of those nations.

"Australia remains ready to arrange a future visit by the special rapporteur," the spokesman said.

Greens senator Sarah Hanson-Young said the announcement highlighted Australia's immigration shortcomings on a global stage.

"The lack of transparency, the lack of free and unfettered access means that Australia is continuing to keep all of their operations in the dark, hidden even not just from the Australian media, but now from the United Nations itself," she said.

"The Government should guarantee that any staff or professionals working inside Australia's detention camps can speak openly to the United Nations, without fear of persecution, prosecution, or indeed the two-year jail sentence."