Two men convicted of murdering the Iranian asylum seeker Reza Barati in Australia’s offshore detention centre on Manus Island will serve five years in prison for the crime.

Louie Efi and Joshua Kaluvia, both 29, were convicted in Papua New Guinea’s supreme court and sentenced to 10 years in jail, with five years suspended.

Taking into account time served, both men will be free in a little over three years.

The court heard that during riots in the Australian-run detention centre on 17 February 2014, Kaluvia twice hit Barati with a length of timber spiked with nails and Efi dropped a rock on his head as he lay on the floor.

But, in sentencing, Justice Nicholas Kirriwom said the men received shorter prison terms because others were also involved in killing Barati. Those people had not yet been charged, he said.

Kirriwom said his decision also took into account that the prosecution’s case relied on the evidence of one main witness.

Kaluvia and Efi pleaded not guilty and maintained their innocence throughout the trial.

In an interview from Lorengau prison in August, they told Guardian Australia they were being “set up”.

“They want to convict us so that nobody else, no Australians or New Zealanders, who are responsible, have to face justice,” Kaluvia said. “We have to take the blame for them because we are PNG. They think we don’t matter.”

Kaluvia fled Manus when he was first named as a suspect. Friends told police he was dead but he was later found in New Britain province. He escaped custody in March but was rearrested after two weeks. Manus police were not informed of his escape for more than a week.

Manus MP Ron Knight said the two Manussian men had been forced to bear the total punishment for crimes others had also committed.

“Our people must see that justice runs both ways,” he said. “We accept that Louie and Joshua have been found guilty, now they must bring in the others who are also culpable.

“We now demand that the two expatriates involved and the others identified be immediately arrested and charged for their part. We believe the two expats were most culpable as they were the last persons seen kicking Reza in the head with safety boots.”

The alleged impunity of expatriates working on Manus has been a long-running sore point with Manussians. In July, three Wilson Security guards accused of drugging and raping a local woman were hurriedly flown off the island and out of the country before they could be interviewed.

In September the then prime minister, Tony Abbott, said Australia would cooperate with PNG in the investigation but the three men have not returned to the country.

Another Australian man employed by Wilson Security was accused of robbing a hotel and crashing a car in December but was flown off the island before police could question him.

Controversy has surrounded the prosecution of Kaluvia and Efi since they were first arrested.

The court was pwhiresented with evidence that up to 15 people, including New Zealand and Australian guards, were involved in attacking Barati before he died.

In the Australian government’s report into the riots in the Manus detention centre, the retired public servant Robert Cornall described Barati as “a very gentle man” who was not involved in the unrest.

Witnesses say on the night of 17 February 2014, as violence in Mike compound reached its peak, Barati was seen running up a flight of stairs to his upper-floor room, pursued by several guards.

At the top of the stairs, a man not in uniform – identified in several witness statements and the Cornall report as Kaluvia – stood carrying a large piece of timber with nails sticking out of the end.

At the top of the stairs, the man identified as Kaluvia allegedly struck Barati in the head with the timber, shouting, “Fuck you, motherfucker.”

Kaluvia allegedly struck Barati again, knocking him to the ground.

One witness statement provided to police said: “Reza Barati was bleeding very heavily from the injury on head. I saw Reza Barati was still alive at that time when he was lying on the wire floor. The G4S guards who were chasing him from behind reached him and kicked him [Barati] on his head with their boots. I saw about a total of 13 G4S local officers and two expatriate officers kicked Reza Barati in his head with their boots. He was putting up his hands trying to block the blows from the boots.

“I then saw this man who was a G4S guard (local) with one eye. He held on to a stone, which was about 30cm wide and 50cm in height. Saw him lifted the stone up with both hands above his head and threw it very hard on Reza Barati’s head while he was still lying on the wire floor. I think at that time when he threw the stone Reza died.”

The Cornall report names Efi, who has only one eye, as dropping the rock onto Barati’s head.

In an investigation in Barati’s death on Manus Island last year, Guardian Australia independently spoke to two further witnesses to Barati’s death.

Both gave identical accounts: that Barati was chased up the stairs and hit twice with the piece of wood before being kicked by several guards, local and expatriate, as he lay bleeding on the ground. Finally, a large rock was dropped or thrown onto his head.

“It was locals and it was expats, they attacked him as he was going up the stairs,” said one man, who did not want to be identified for fear of retaliation from people on Manus Island.

“They hit him and they kicked him with their boots. And they dropped a rock on his head. We watched all of this, we saw him die.”

The man told Guardian Australia he feared giving evidence in the men’s trial for fear of reprisals from other guards and local people. He said he had been told he would be killed if he testified.

A medical officer who treated Barati told the Cornall inquiry it was immediately clear the asylum seeker would not survive his catastrophic head injury.

“Mr Barati’s head was shattered by a crack on the left side of his skull … he also had facial abrasions and knocks indicating he has received a more general beating (not just the blow to the skull).”

Barati’s catastrophic brain injury caused cardiac arrest.

Cornall concluded: “Mr Barati suffered a severe brain injury caused by a brutal beating by several assailants and died a few hours later.”

Seventy asylum seekers were injured in three days of the rioting in February 2014, including one man who was shot and another who had his throat slit. The riots were “eminently foreseeable”, according to an Australian Senate inquiry, and caused by a failure to process asylum claims.

The inquiry found the Australian government – which labelled the incident a “disturbance” – failed in its duty to protect asylum seekers.

David Yapu, the police commander of Manus Province, lauded an “ambitious police investigation spanning over three years” leading to the conviction, despite two escapes by Kaluvia.

Yapu said the convictions were “quite relieving” and would “undoubtedly reflect positively” on the PNG justice system.