The two Papua New Guinean men accused of killing asylum seeker Reza Barati on Manus Island have told a court the main witness against them signed a false statement written by "Australian guards".

Joshu Kaluvia and Louie Efi are charged with wilful murder over Mr Barati's death. Both have pleaded not guilty to the charges.

They are accused of hitting Mr Barati with a piece of wood and a large rock respectively, causing him to die from a serious head wound on February 17 last year.

Mr Barati's roommate at the Manus Island detention centre, Benham Satah, said he watched the pair attack the Iranian asylum seeker.

The two men told the PNG National Court they spoke with Mr Satah at the prison in Lorengau, the main town on Manus Island, after they saw a witness statement from him in their court file.

But Mr Kaluvia said Mr Satah told them he had not written the statement.

"He said it was a 'frame-up' by the Australian guards and he said what the story that was written down, he must base it on that," Mr Kaluvia told the court.

"He said if he doesn't do it, they will bribe the locals to attack him."

Mr Kaluvia, who was a recreation worker with the Salvation Army, told the court he had not entered the centre until after the violence had ceased.

He also denied sending text messages which appeared to admit to the murder, saying he did not have his phone after the incident.

One message, allegedly to Mr Kaluvia's girlfriend, read: "On the TV News most of the asylum seekers are dead. We killed them, especially me and my friend."

Mr Efi, who was a security guard with centre managers G4S, said he had been a "static guard" who played no role in putting down the violence.

But he had sought treatment for cuts to his hand sustained that evening.

His brother, Emmanual Efi, said outside court that the way his brother became a suspect in the murder was suspicious.

"After the incident he complained about his salary and the cuts [for which] he was supposed to be compensated and then the next thing, all the fingers were pointing on him," he said.

Witness says he was threatened

Earlier, Mr Satah had told the court he saw Mr Kaluvia and Mr Efi at the Lorengau Hospital, not the prison.

"They told me 'if you don't withdraw your affidavit we will do what we did to Reza'," he said.

A witness in the trial over the death of Reza Barati (pictured) says he saw two men attack the asylum seeker. ( AAP: Eoin Blackwell )

Mr Satah said he was scared when he was confronted by the two accused.

"I am witnessing the death of my roommate and Wilson Security exposed me to the killers of my roommate in the Lorengau Hospital," he said.

Mr Satah was brought to court under special police guard, but said new measures introduced to protect him in the Australian-run detention centre actually placed him in more danger.

"They have isolated me from the others, they have isolated me from my friends," he said.

"I have to live in fear, please don't send me back to that life."

Mr Satah had told the court he was scared to testify against Mr Kaluvia and Mr Efi but on Monday identified both of them and described how they had attacked Mr Barati.

He said Mr Kaluvia began the attack by hitting Mr Barati on the head twice with a large piece of wood with nails sticking out of it.

Other people kicked Mr Barati in the head after he fell down, Mr Satah said, before Mr Efi dropped a large rock on his head.

Both Mr Kaluvia and Mr Efi rejected Mr Satah's account.

The court is yet to hear final submissions in the case before Judge Nicholas Kirriwom retires to make a decision.