A Manus Island security guard warned colleagues that the detention centre was ''totally unprepared'' for any major incident, months before an Iranian asylum seeker was killed in violent clashes.
Paul Skillen, who worked as a G4S security supervisor at the centre, emailed colleagues in November expressing concerns that poorly trained workers were staffing the centre, which was ''a tinderbox ready to ignite''.
The emails have been submitted to a Senate inquiry set up to investigate events at the centre in February, which left 62 asylum seekers injured, one with gunshot wounds.
Reza Barati, a 23-year-old Iranian asylum seeker, was killed by several blows to his head during the violent confrontations.
In his emails, Mr Skillen warned that local Papua New Guinea workers were being assigned to high-risk roles with no experience and limited training.
He said the entire structure of the incident response team was not working and needed revision. ''I understand there are some plans afoot. However, I feel the centre is currently a tinderbox ready to ignite,'' he wrote.
''The intelligence being received at the moment, I am led to understand, supports my thoughts. In the event that the tinderbox ignites, the centre will be totally unprepared, and members of the IRT and other staff will undoubtedly get hurt, some badly.''
Mr Skillen said questions about the consistency of the training and leadership ''leaves both G4S as a company, and the individuals concerned, liable should anything untoward occur''.
On Friday, a spokesman for G4S told Fairfax Media the company rejected the allegations in the emails ''and we will be addressing them in our submission to the Senate inquiry''.
In a second email that was part of his submission, Mr Skillen wrote that basic crime-scene preservation measures were not in force at the facility after a hanging attempt in one of the compounds.
He described having to remove evidence in plastic garbage bags and said there was no control over who entered and handled items in an area that was potentially a crime scene.
''If we are unlucky enough to have a death in custody, we may all end up before a PNG coroner, and these simple steps would assist us and the PNG constabulary greatly,'' he wrote.
Immigration Minister Scott Morrison did not respond to specific questions about Mr Skillen's submission.
A spokeswoman for Mr Morrison said in a statement the government was assisting PNG in finalising its police investigations.
''Ministers agreed that all relevant information from the independent administrative reviews would be handed over to the police investigation to enable swift prosecutions,'' she said.
