Two men found guilty of murdering asylum seeker Reza Barati in Australia's offshore detention centre on Manus Island have been sentenced to 10 years' jail, with five of those years suspended.

Key points: Two men will be released in just over three years over riot death

Two men will be released in just over three years over riot death Mr Barati hit repeatedly with piece of wood before being hit in head with rock

Mr Barati hit repeatedly with piece of wood before being hit in head with rock Sentence lower because others involved in the killing not charged

Sentence lower because others involved in the killing not charged Senate inquiry found Australian Government failed to protect asylum seekers

With time already served, Joshua Kaluvia and Louie Efi will be released from prison in just over three years.

Papua New Guinea's National Court found the men murdered Mr Barati during a riot at the Manus Island detention centre in February 2014.

The court heard Kaluvia hit the 23-year-old Iranian in the head repeatedly with a piece of wood with a nail in the end of it, before Efi dropped a large rock on Mr Barati's head.

Efi and Kaluvia were charged with wilful murder, but sentencing judge Nicholas Kirriwom found them guilty of the lower charge of murder.

He told the court he applied for the lower charge and suspended part of their sentence because there were other people involved in the killing who had not been charged.

Judge Kirriwom also told the court his decision took into account that the prosecution relied on only one main witness.

"I bear in mind that in sentencing these two prisoners, I do not make them 'guinea pigs' to bear the brunt of punishment for those who are not here and have not been prosecuted," he said.

Last year, the court heard police had tried to question one Australian, named Paul, and one New Zealander, named Anton, who were working at the detention centre and were allegedly involved in Mr Barati's death.

But police told the court they had not received any cooperation in bringing the men back to PNG.

'Many people' involved in killing asylum seeker

Detainees at the centre said there were many people involved in the killing of Mr Barati, as well as other acts of violence during the riot, who had not been charged.

The main trial witness, Benham Satah, said he had received threats from other guards at the centre and from the defendants themselves, when he was taken to the hospital in the town of Lorengau.

"They told me, 'If you don't withdraw your affidavit we will do to you what we did to Reza'," he told the court last year.

A former Salvation Army worker, Kaluvia escaped from jail on Manus Island on March 28, but was recaptured just over two weeks later to face court.

Police had struggled to find Kaluvia when he was first named as a suspect in the case after his friends initially told them he was dead.

He was later arrested on the island of New Britain.

A Senate inquiry in December 2014 found the cause of the riot, which injured 70 asylum seekers, to be a failure to process asylum seeker claims, stating the violence was "eminently foreseeable".

It also found the Australian Government — which labelled the incident as a "disturbance" — failed in its duty to protect asylum seekers, including Mr Barati.