The United Nations has postponed a planned visit to Australia because the federal government cannot guarantee legal immunity to detention centre workers who discuss asylum seekers and migrants.

The United Nations' Special Rapporteur on the human rights of migrants, Canada's Francois Crepeau, was due to visit Australia on Sunday for about two weeks to investigate the plight of migrants and asylum seekers in offshore detention centres on Nauru and Manus Island, following an invitation from the federal government.

But Mr Crepeau said in a statement that the Border Force Act, which makes it a crime for immigration and border protection workers to disclose information about offshore detention centres, "serves to discourage people from fully disclosing information relevant to my mandate".

Under the law, such people face up to two years in prison for recording or disclosing information they obtain from their work.

"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."

It was impossible for Mr Crepeau to carry out his visit as an independent expert for the UN because the Australian government "was not prepared" to meet his request for a written guarantee that anyone he met during his visit would not risk being intimidated or face imprisonment under the law.

A spokesman for Immigration Minister Peter Dutton described the postponement as "disappointing and unfortunate".

"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 declined to say whether the government would consider offering exemptions to the secrecy provisions of the Australian Border Force Act, saying: "The Special Rapporteur was briefed on the responsibilities and obligations of personnel under relevant Australian law.

"Australia remains ready to arrange a future visit by the Special Rapporteur."

Mr Crepeau said Australia had also denied his repeated requests for full access to offshore detention centres since March.

"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," he said.

The Special Rapporteur said he had been planning the visit with the Australian government since January.

Mr Dutton's spokesman said the Department of Immigration had worked closely with Mr Crepeau's office to organise a programme for his visit, which was to include visits to detention centres, and meetings with key government officials and service providers.

But he said the government had no role in organising access to offshore detention centres: "Access to Regional Processing Centres in Papua New Guinea and Nauru is the responsibility of these sovereign nations and needs to be addressed with their governments."

Organisations including the Australian Human Rights Commission, UNHCR and Commonwealth Ombudsman, had visited both on and offshore detention centres "without the need to respond in this way," he said.

The Human Rights Law Centre's executive director, Hugh de Kretser, said the cancelled visit was "unprecedented for a western liberal democracy".

"This is extremely damaging for Australia's reputation – particularly when our human rights record will be reviewed at the UN in November and we're seeking election to the UN Human Rights Council in 2018. It's extremely damaging to our ability to advance our national interest on the world stage," said Mr de Kretser.

It was also a "huge missed opportunity" for newly-appointed Prime Minister Malcolm Turnbull to pursue a "more constructive relationship with the UN".

"We urge the Australian Government to urgently provide the necessary assurances to the Special Rapporteur to enable the official visit to take place at a future date."

Doctors, and humanitarian workers have previously criticised the Border Force Act which was passed earlier this year with the support of Labor, saying it prevents proper public scrutiny of detention centres in line with their duty of care to asylum seekers.

The government has dismissed such claims, saying a separate federal law ensured officials were protected in making "public interest disclosures". But it is unclear which health or medical professionals would be required to comply with the new secrecy provisions.

Under the law, workers can only release such information legally if they have permission from the secretary of the department, if they are authorised by law, or if a court or tribunal orders or directs them to do so. The secretary would have to be satisfied that the information would help the person to perform their duties or powers to give them permission to release it.