Security firm G4S has revealed it warned the Australian Government that Papua New Guinea police were unreliable and unpredictable, in the days leading up to the deadly Manus Island detention centre riot.

Local officers reportedly fired dozens of shots during the incident and one man was killed and dozens more injured during violent clashes at centre in February.

In its submission to a Senate inquiry, G4S says tension had been growing at the centre for weeks over the extreme delays in processing claims and the lack of information being given to detainees.

The company says it told the Immigration Department in early February that it was preparing for violent protests, in conjunction with the PNG police, but warned the local force was an unknown quantity and may resort to gas and firearms if provoked.

G4S says it had also been warning the department for months that fencing and other security infrastructure was inadequate.

In March, a spokesperson for Immigration Minister Scott Morrison said there were three investigations underway into the Manus Island incident, including a full independent inquiry headed by former senior public servant Robert Cornall.