The UN Special Rapporteur on the Human Rights of Migrants says a lack of cooperation from the Australian government has forced him to postpone an official fact-finding mission.
Francois Crépeau was scheduled to visit Australia for two weeks to gather first-hand information about the situation of asylum seekers detained in Papua New Guinea (PNG) and Nauru, on the invite of the Australian government.
But, in a statement, he said the new Border Force Act, which sanctions detention centre workers who disclose information about detention centres with up to two years in prison, would impact his visit as "it serves to discourage people from fully disclosing information relevant to my mandate".
"This threat of reprisals with persons who would want to cooperate with me on the occasion of this official visit is unacceptable," Mr Crépeau said.
"The [Border Force] 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 Crépeau said Canberra was not prepared to give a written guarantee that anyone he met during his visit would not face judicial sanction, nor was it prepared to facilitate full access to offshore detention 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," he said.
Human Rights Law Centre executive director Hugh de Kretser said the postponement of Mr Crépeau's visit highlighted threats to whistleblowers in Australia's detention regime.
"It's extraordinary that the Australian government couldn't assure the UN expert that people he spoke to wouldn't be exposed to prosecution under the Border Force Act," said Mr de Kretser, who was due to meet with Mr Crépeau in Melbourne this weekend.
"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.
"Not only did Australia fail to provide the standard assurances on reprisals, it refused to facilitate access to offshore detention centres that it funds and effectively controls."
Mr de Kretser said the incident was a "huge missed opportunity" for Australia's new prime minister, Malcolm Turnbull, to pursue a more constructive relationship with the UN, and urged him to provide the necessary assurances to enable the special rapporteur's visit to take place.
Since its introduction in July, the Border Force Act has faced sustained criticism from civil society groups and former detention staff, who have labelled it an attempt to gag whistleblowers from speaking out about abuse.