The violent clashes on Manus Island that left an Iranian asylum seeker dead happened after local G4S descended on protesting asylum-seekers, a preliminary police briefing has indicated.

A Papua New Guinean police spokesman has told Fairfax Media that 23-year-old Reza Barati was killed by multiple blows to the head, probably from a piece of timber.

In a briefing to  Fairfax Media, drawing on several police reports, the spokesman said the autopsy, carried out on Monday, found that the blows caused a blood clot on the man's brain.

''The object used we suspect was a heavy (piece of) timber or wood or some such object,'' the spokesman said. ''There were no other wounds on the body.''

The spokesman said the findings were preliminary but indicated that local G4S guards had carried out the violent suppression of the protest. PNG police and local villagers did not appear to be involved, he said.

He said that the Australian and PNG immigration chiefs on the island had on Monday ''liaised with the heads of the service provider in the centre to organise the employers for police to obtain statements from them, especially from the local employees hired by the G4S security company''.

But nobody was being ruled out, he stressed, as investigations were continuing. He said no arrests had yet been made.

The spokesman said that G4S management in the centre had stood down all the local security guards. The police had been forced to get them back to the centre for interviews, he said.

Those who came from outside Manus Island were currently prohibited from going home. The spokesman could not say whether they were currently working in the centre.

The situation was ''tense, but under control'', he said.

The police had begun interviewing local G4S workers on Monday and were still carrying out the interviews, the spokesman said.

Mr Kauba said about seven or eight officers were investigating "on the ground" and he expected the case to "take all week".

The police spokesman said reports had been submitted by the local police chief Alex N'Drasal on the events.

"Mobile squad" police officers were providing written statements and also being interviewed, the spokesman said.

But initial reports suggested they had not been involved in the violence.

''None of the transferees actually came out of the perimeter fence. The G4S guards and transferees were all fighting inside the compound,'' he said.

However, he did not explain how one of the detainees came to be shot in the buttocks. G4S guards are not armed but PNG police are.

Warning shots had been fired, the police spokesman said. These had likely calmed the situation, he said. "Otherwise, the place would have been burnt to the ground."

Deputy police commissioner Simon Kauba told Fairfax Media: "The investigation is ongoing. We've employed people in Port Moresby and Manus."

Mr Kauba said about seven or eight officers were investigating ''on the ground'' and he expected the case to ''take all week''.

There was confusion about the help being given by Australians. Mr Kauba said Australian assistance was "not necessary" and had not been requested.

But a spokeswoman for the AFP said the Royal Papuan New Guinea Constabulary (RPNGC) had asked for help with "the coronial of the deceased Iranian being conducted by the PNG authorities".

The spokeswoman said two Victorian Institute of Forensic Medicine officers would travel to Port Moresby to support the PNG coroner with the autopsy.

The local PNG Post Courier reported that Mr Kauba had said that locals and PNG police had been "cleared" of the investigation.

In Parliament meanwhile, the House of Representatives passed a motion admonishing shadow defence minister Stephen Conroy for accusing Lieutenant-General Angus Campbell, the head of the government's Operation Sovereign Borders, of being engaged in a political cover-up following the death.

Foreign Minister Julie Bishop demanded that Labor leader Bill Shorten sack Senator Conory, describing him as "this dog of war", but Mr Shorten said the senator's withdrawal of the remark spoke for itself.

Tasmanian independent Andrew Wilkie, a former classmate of General Campbell, moved the motion, saying: "I think a line was crossed yesterday when the Opposition defence spokesperson called into question the integrity of General Campbell".

Senator Conroy defended himself, saying: "We've seen tragic circumstances arising on Manus Island as a result of the Government's actions and there needs to be a full account.

On Wednesday night it was reported that up to 60 detainees in the family compound on Christmas Island, including mothers and teenage boys, were staging a hunger strike following Reza Barati's death, refugee advocates say.

"It's been going for about three days now," Ian Rintoul, from the Refugee Action Coalition, said.

"It started out of a vigil and a protest at the death of Reza Barati on Manus."

Mr Rintoul, who said he had spoken directly to two of the hunger strikers, had not been told how long the asylum seekers planned to refuse food.

Around 150 people were also staging a daily protest march around the compound each day, he said.

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