The firm pleaded with the department to defuse the tensions in the camp by speeding up refugee assessments and talking to the asylum-seekers.

At least one asylum-seeker was shot during the night of violence on February 17 in which 23-year-old Iranian asylum-seeker Reza Barati was killed.

Witness accounts and photographs indicate PNG police fired dozens of shots, some of them at chest or head height. Locally employed G4S guards, who are believed to have carried out much of the violence, were armed with makeshift weapons, not guns.

In a letter dated February 4, Sven Straub, G4S's acting managing director for the southern Pacific region, told Immigration head Martin Bowles the security firm was planning with PNG police to prepare for possible violence.

This included Manus Island police and the feared "mobile squad" from mainland PNG, which has a reputation for brutality.

''However, they have not proved reliable in the past, and our concern is that they may resort to gas and then firearms if they are provoked,'' Mr Straub wrote. ''In short, they are an unknown quantity in a serious incident.''

The letter and other documents have been submitted to a Senate inquiry into the riots.

Mr Straub said the lack of progress on refugee assessments and scarce information given to detainees had provoked protests, and these were expected to escalate to violence.

He said the asylum-seekers appeared to ''understand they will not go to Australia now'' but were angry that ''there is no processing for refugee status determination taking place''.

''We have repeatedly asked the Island immigration staff (both Australian and PNG) to engage with transferees, to little avail, although a series of Q&A sessions has now been agreed.

''In short, it is our view the situation could be easily defused if transferees can be provided reassurance on timelines for processing of their claims and if PNG (Immigration) commenced processing claims again.''

Only in recent days have refugee decisions been handed down on Manus Island. In February, there were mixed signals from Port Moresby on whether refugees would be resettled in PNG.

Greens senator Sarah Hanson-Young, who instigated the Senate inquiry, said the government ''wilfully ignored the warning signs and they allowed the toxic culture to breed and grow''.

She said the G4S submission also highlighted the unsuitability of Manus Island as an offshore detention centre, citing the requirement that 50 per cent of the staff had to be PNG nationals, who were ill-equipped to deal with the challenges they faced, and PNG’s inability to process refugee claims.

A spokeswoman for Immigration Minister Scott Morrison said the minister would not pre-empt a review by former senior public servant Robert Cornall, which would ''consider all relevant information in arriving at its findings as, we understand, will the police investigation''.