Australian authorities insisted that their controversial Manus Island detention centre will stay open despite fatal clashes between asylum-seekers and Papua New Guinea police.

One man was killed and 76 injured during a second night of violence at the camp on Monday.

Of the 12 seriously injured asylum-seekers, two were still in a critical condition in an Australian hospital - one with a fractured skull and another with a gunshot wound.

Thirty-five asylum-seekers broke out of the same detention centre on Sunday night as unrest flared about their fate under the Australian government's hardline policies.

What sparked Monday night's unrest was unclear, with the Refugee Action Coalition saying PNG police and locals had stormed the centre.

Spokesman Ian Rintoul said inmates inside the camp told him that gangs of police and locals armed with machetes, pipes, sticks and stones had roamed from compound to compound within the camp attacking asylum-seekers.

He said those inside had been left defenceless after guards were evacuated.

"This wasn't a breakout ... the vast majority were attacked inside," Mr Rintoul said.

"It must be clear now that asylum-seekers cannot live safely on Manus Island."

But notorious private contractor G4S, which operates the camp, denied that any outsider had breached the perimeter.

Australian Immigration Minister Scott Morrison initially said the clashes occurred outside the camp but later admitted he didn't know where the injuries had been inflicted.

He said the unrest began when detainees pushed down internal barriers in the camp and the riot "escalated quickly with several hundred transferees" rushing to tear down external fencing.

Security staff used shields to push them back while PNG police mobilised to capture those who had escaped, firing at least two shots, Mr Morrison claimed.

"This was a very dangerous situation where people decided to take themselves outside the centre, placing themselves at great risk."

Amnesty International condemned the camp, joining Australian opposition parties in calling for an independent investigation of the violence.