Asylum seekers will be invited to give their witness accounts of the recent deadly violence on Manus Island to a parliamentary committee that will free public servants and contractors from confidentiality clauses in their employment contracts.

Having persuaded the Labor Party to back the inquiry, Greens senator Sarah Hanson-Young will push for it to conduct hearings inside the Australian-run detention centre where more than 60 asylum seekers were injured and one died last month.

Detainees have so far spoken, on the promise of anonymity, of local security contractors and others invading the centre and savagely beating and slashing detainees who tried to hide in the bedrooms after a non-violent protest.

Other staff have told Fairfax Media they know the identity of the man who killed Iranian detainee Reza Barati.

''I think it's going to be difficult to run a genuine inquiry unless we get there,'' Senator Hanson-Young said after agreeing with Labor on wide-ranging terms of reference for the probe.

The combined numbers of Labor and the Greens in the Senate ensure the inquiry will go ahead and will be dominated by non-government MPs. It will be chaired by Greens senator Penny Wright.

Under its terms of reference, the inquiry will review the conduct of the Minister for Immigration and Border Protection, Scott Morrison, ''before, during and after the incident''.

The inquiry will take evidence in April, after the inquiry set up by the government has concluded, and is expected to call asylum seekers, staff from G4S - the company that has been running the centre - and the Salvation Army, Papua New Guinean police and Australian officials.

Witnesses would have parliamentary privilege to speak openly and not be bound by confidentiality terms in their employment contracts, Senator Hanson-Young said.

''It is very clear that a lot of people who have witnessed what happened and want to speak out, but are scared of their confidentiality agreements and intimidated by the department,'' she told Fairfax Media.

''I spoke to an interpreter today who has been receiving phone calls from the department threatening her if she says anything.''

If the inquiry was refused permission to conduct a hearing within the centre, Senator Hanson-Young said detainees would be able to give evidence by teleconference.

She dismissed the inquiry ordered by Mr Morrison as compromised because it was not arm's-length from government; the government would decide whether its findings were released in their entirety, and there were criticisms of the former public servant leading the inquiry, Robert Cornall.

Mr Cornall investigated allegations of sexual and other serious assaults at the centre last year, concluding claims of detainees being sexually abused, raped and tortured with the full knowledge of staff were not true and that other allegations were exaggerated.

''This inquiry will be the only independent investigation into Reza Barati's death and what happened on Manus Island,'' Senator Hanson-Young said.

Labor's immigration spokesman, Richard Marles, backed the inquiry, saying: ''It is vital we get to the bottom of what went wrong on the minister's watch at Manus Island. So far, Mr Morrison has consistently failed to provide an appropriate response to critical questions relating to events between February 16-18."

A spokesman for Mr Morrison said the government's review would ''now benefit from the participation of the government of PNG''.

''In addition, the PNG government has established a police investigation and a coronial inquiry to ensure that any potential breaches of PNG laws are appropriately dealt with through their justice system.''

Manus Island police chief Alex N'Drasal has told Fairfax Media that he expected his investigators would soon arrest more than one suspect in relation to the death of Mr Barati.

''We're still continuing the investigations, but there are indications that we will probably soon arrest someone in relation to that,'' he said.

He said his investigators had the names of ''a few suspects''. He could not give a precise number or say exactly when the arrests would happen.

Mr N'Drasal also stood by previous claims by PNG police that their officers remained outside the detention centre and took no part in the violence.

What the inquiry aims to establish

• A chronology of events
• The sequence of events that led to Reza Barati's death
• The involvement of the Immigration Department
• How the PNG police, military and civilians were involved
• Any documents involving staff, employees, contractors and sub-contractors involved
• Any communications between the Minister for Immigration and Border Protection and the PNG government regarding the employment of contractors, sub-contractors and service providers
• Australia's duty of care obligations and responsibilities
• Refugee processing and resettlement arrangements in PNG

Follow us on Twitter