A witness to the violence at the Manus Island detention centre on Monday night says guards attacked asylum seekers with sticks and iron bars.

His is just one of a number of witness accounts emerging following this week's deadly violence.

An Iranian national was killed and 13 people were seriously injured after two consecutive nights of unrest at the centre on Sunday and Monday.

PNG authorities and Australia's Immigration Department are conducting investigations.

The commander of Operation Sovereign Borders, Lieutenant General Angus Campbell, has been dispatched to Manus Island as well as 51 additional security staff.

In a statement, G4S says it takes the allegations seriously and does not tolerate violent or abusive behaviour from staff.

A man who saw the violence on Monday night, but does not want to be identified, says Papua New Guinean guards employed by the G4S security company running the centre became angry when asylum seekers shouted insults about their country and family members.

The witness says he watched PNG guards beat detainees with sticks, iron bars and rubber hoses as well as fists, boots and shields.

"When they pulled them outside they started beating them with the sticks... some of them with sticks and some of them with all these hose, rubber hose and pipes," he said.

The witness says some asylum seekers tried to escape the violence by going into rooms, but were taken out and assaulted.

"Once they got inside the room they were trying to hide themselves... they don't want to get hurt. That's why they're trying to lock them up," he said.

"But the G4S went in, opened the room and pulled them out and [belted] all of them."

Interpreter describes detainees with horrific injuries

Iranian-Australian freelance interpreter Azita Bokan was contracted to work on Manus Island for the Department of Immigration last week.

She was on Manus Island during the violent clashes on both nights.

Ms Bokan claims she was dismissed on Sunday after trying to intervene in a scuffle between detainees and G4S staff.

On Monday night she stood on the roof of the staff housing facility several hundred metres away from the compound and watched the unrest.

She has described seeing detainees with horrific injuries after a deadly clash involving large rocks and machetes.

"I haven't had sleep in three nights. I'm so all over the place, myself. I've seen too much and I keep having flashbacks," she told the ABC.

"The injuries were horrendous. It was blood everywhere. Throats were cut. It was heads crushed by the rocks."

Ms Bokan claims she saw uniformed G4S guards jump over the fence towards detainees, but could not see who was wielding the rocks and knives because it was too dark and she was too far away.

"Everyone was running everywhere saying, 'help us', shouting, yelling, lots of yelling was happening, and the people say, 'they're killing us, they're killing us'," she said.

Interactive: Australia’s offshore immigration centres Find out more about Australia’s operations on Christmas Island, Manus Island and Nauru.

Ms Bokan says the violence she saw is like nothing she has ever witnessed.

"What is in Australia, it does not apply there. I was in Darwin, I was in other detention centres. Nothing compares to what you see in Manus Island," she said.

Ms Bokan says she saw detainees trying to defend themselves by holding up plastic chairs.

"I never would've thought under Australia's name we do anything like that. I still keep Australia highly up," she said.

"I do not believe Australians are about this. We do not accept this in our names."

Immigration Minister Scott Morrison says he has had no verification of anything of the nature described by Ms Bokan.

"Those investigations and the collection of that information is under way, and I would caution against such speculation until these matters can be properly confirmed," he told the ABC.

"I imagine there will be all sorts of interviews undertaken from people who were there and the various other authorities and that goes well beyond the perimeter fence.

"It goes to establishing the facts of what occurred that night. I think this is going to be a very difficult process."

Guards say they were outnumbered by asylum seekers

Two G4S guards at the centre have also spoken out for the first time about the two nights of violence.

The men, who do not want to be identified, say the worst of the violence on Monday night occurred after a riot squad was outnumbered by asylum seekers and guards rushed in to help.

They denied reports that machetes or bush knives were used in the violence.

They admitted to punching detainees and aggressively tackling asylum seekers trying to escape but say the force was reasonable given the situation.

One of the Papuan New Guinean guards, employed by G4S, says around 20 members of the company's Incident Response Team dressed in riot gear tried to contain the protests.

The guard says the riot team failed to stop the asylum seekers shouting insults and throwing stones and were outnumbered by several hundred detainees.

"They were swearing at us... swearing 'f*** PNG' and something like that and they don't like us, so they start throwing stones and start pushing gates," he said.

"The serious violence happened when we went in to contain the situation. The clients outnumbered them and they can't contain the situation.

"So what we do is we all G4S local nationals and expats we just rushed in to save our counterparts. So that's where the trouble begin and the fight started."

The guard says Papua New Guinean and Australian staff working for G4S then entered the compound and serious fighting erupted.

"They are using their hands only, their hands to hit them... the clients," he said.

The second guard says force was used to contain the situation.

"There were too many of them in the compound because each compound contains about 400 clients in Oscar compound. About 50 per cent of the clients they were fighting, throwing things at us. So some of us were hit and we retaliated," he said.

"I have to save my friends from the clients because they were not playing."

G4S vows to cooperate with Government review

G4S said in a statement it is concerned by allegations that PNG nationals working for the company may have been involved in violence against transferees on Manus Island.

"Our duty of care to the transferees has always been and remains our highest priority," the statement said.

"Our personnel on duty during the disturbances acted with courage, strength and determination to protect those in our care.

"Importantly G4S guards at the Manus Island Centre are not armed and do not carry weapons of any sort.

"We support the investigation of the incident that was announced by Minister for Immigration and Border Protection, Scott Morrison, and we will fully cooperate with that investigation."