The Australian Government has serious questions to answer over its failure to guarantee the safety of a key witness to the murder of asylum seeker Reza Barati, according to a United Nations official.

Key points: Two men who witnessed Barati murder allegedly tortured, threatened with rape

Two men who witnessed Barati murder allegedly tortured, threatened with rape One witness still fears for his life despite being promised protection

One witness still fears for his life despite being promised protection The other has returned to Iran over fears for his safety

The other has returned to Iran over fears for his safety UN special rapporteur questions Government response six months on

The key witness, Kurdish asylum seeker Benham Satah, has told the ABC's AM program that he remains in fear of his life.

He is one of two men allegedly tortured inside a secret compound in the Manus Island Regional Processing Centre.

Mr Satah was one of two roommates of Barati. He spoke to the ABC from inside the Manus Island detention centre, where he said he faced daily intimidation from PNG guards despite being promised witness protection by local authorities.

"They want me dead," Mr Satah said.

"They have increased their pressure every day since Reza died. How many times can I survive more?"

UN special rapporteur Juan Mendez detailed the torture of Mr Satah and another Iranian man in a report tabled at the UN's Human Rights Council earlier this year.

The two men were allegedly tortured by local PNG guards inside the Chauka compound on Manus Island and threatened with rape unless they retracted sworn statements that detailed events on the night of Barati's murder.

Mr Mendez told the ABC that more than six months since the report was tabled, he was not satisfied with the Australian Government's response to the torture allegations.

He also said Australia had not done enough to protect key witnesses to the Barati murder.

"I think it's never too late for Australia to do what it should have done some time ago," Mr Mendez said from Washington.

"Providing witness protection is something that should have been done on a timely basis, but it can still be done and I'm hoping that the Australian Government will decide to do so."

'I have seen how they kill people'

Until late last month when he was forcibly escorted to a Manus Island court, Mr Satah had been refusing to give evidence against the two men accused of the Barati murder, PNG locals Joshua Kaluvia and Louie Efi.

Two expatriates, an Australian national and a New Zealand national, who were also allegedly responsible for Barati's murder, have so far escaped justice. The Barati trial is currently adjourned.

"I have seen how they kill people," Mr Satah said.

"I can't sleep well. My legs never stop shaking and I feel pain in my chest and my head all the time."

Detention centre workers Louie Efi (left) and Joshua Kaluvia (right) are accused of murdering Reza Barati. ( ABC: Eric Tlozek )

Last month Mr Satah was promised protection by the PNG judge presiding in the Barati murder case and the operators of the Manus Island detention centre, and eventually gave evidence in court.

He said he spent the next 15 days in a special area called SAA compound which was isolated from the local guards. But then he said a group came into the compound to intimidate him.

"Eight locals to come to SAA, including one of the locals who was involved in Reza Barati's murder," Mr Satah said.

"They sent them deliberately to frighten me and force me to go back [to Iran]."

Mr Satah said he asked to be moved to another compound, where he is now subject to ongoing intimidation.

"I'm in Foxtrot compound in M block and there is no camera here. Every day many locals come and sit in the back of my room for three or four hours a day. They want to agitate me and force me to go back to my country," he said.

"I don't feel safe here because security already killed my best friend and they proved they can't provide safety for anyone in here and they are trying to agitate me every day by exposing me to locals."

Witness returned to Iran because of safety fears

Mr Mendez said Australia had questions to answer over its response to the Barati murder.

"In this particular case there are very serious questions," he said.

"It's been more than two years since the incident and things like witness protection could have been afforded, things like involvement of Australian contract agents should have been investigated, and we don't see enough results on that.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 4 minutes 51 seconds 4 m 51 s Australia 'has questions to answer' Download 8.9 MB

"These people were tortured because they were witnesses to the murder of another asylum seeker.

"Australia in this case has a responsibility because none of this would have happened but for the policy of creating a detention camp in Manus Island and letting it be run by PNG."

The second man allegedly tortured in Chauka compound, Witness B, was so afraid for his safety that he returned voluntarily to Iran.

"The fact that he returned voluntarily says something about how dangerous it is for people to be in Manus Island," Mr Mendez said.

The Australian Border Force did not respond to detailed questions about the Barati trial, the safety of witnesses, the investigation of torture allegations and punishment of perpetrators and what it was doing to apprehend the two expatriates still at large.

"The court process is a matter for Papua New Guinea authorities," a spokesperson said.

"Australia will continue to offer assistance where required to the PNG authorities in this matter.

"As the matter is before the courts, it would be inappropriate to comment further."