Two asylum seekers in detention on Manus Island say they were forced to retract their eyewitness accounts of violence in February after being beaten and threatened by Australian officials at the centre.
The Australian government says their claims are false.
One of the pair was the room-mate of the murdered Reza Barati, and has identified several staff at the centre as having been involved in the killing.
Their claims have been forwarded to Australian Federal Police and the office of the UN High Commissioner for Human Rights by Perth-based human rights advocate Ben Pynt.
"This is active interference with a police investigation and tampering with witnesses," says Mr Pynt, who is director of human rights advocacy for Humanitarian Research Partners.
Immigration Minister Scott Morrison rejected outright claims that the men have been badly treated.
Mr Pynt also claims three other asylum seekers who nominated to return to Iran were told they could leave only if they withdrew complaints against centre staff.
At least one who has opted to return has been told he will be required to remain in PNG until investigations are complete.
The two asylum seekers allege they were subjected to "cruel, inhuman and degrading treatment" over a number of days last week before they say they agreed to retract their police statements.
They say they were taken to an area of the camp they had not seen before and fed bread and water for three days, sleeping on the muddy ground.
"We were crying and asking what is our fault?" one said in a Facebook post. "They said: 'Because you always object to all of our rules'.'' They claim they were cable-tied to chairs and beaten about the body to avoid noticeable bruises and threatened with rape and murder if they did not retract their statements.
A spokesperson for the minister rejected the claims, repeating that Mr Morrison had been advised that "two men became abusive and aggressive and were moved in accordance with operational policy within the centre".
The two men allege the episode followed a complaint on behalf on Iranian detainees about restrictions on internet access.
"They are terrified of reprisals," says Mr Pynt, who says he has "every reason to believe these men, with whom I have been communicating for months".
He said the two men, who have asked that their names not be published, had asked him to do anything he could to keep them safe.