The former security management firm at the Manus Island detention centre is refusing to be interviewed about February's deadly riots, police in Papua New Guinea say.

Manus Island police are investigating the death of Iranian asylum seeker Reza Berati, who was killed by multiple blows to the head during the riots.

Deputy police commissioner Simon Kauba says the inquiry is nearing completion, but that security firm G4S is proving to be a stumbling block.

"We are at the final stages, we want to tidy a few things where we thought it was vital to interview G4S management and one or two of the asylum seekers before we actually interview the suspects," he said.

"We are unable to get a favourable response from the G4S management."

Commissioner Kauba says the PNG police need to interview several key personnel before they lay charges.

"My team of investigators led by the provincial police commander in Manus are still negotiating, trying to get the sort of assistance that we want from the G4S." he said.

G4S was in charge of security at the centre when the violent riots occurred between February 16 and 18, but handed over operations to Wilson Security last week.

Sixty-two people were injured in the protests, including one man who was shot in the buttocks.

Several G4S staff left Manus Island after the incidents suffering severe post-traumatic stress.

In a statement, the security firm has denied suggestions it is not cooperating with PNG police.

"G4S strongly refutes any suggestion that it has in any way hindered or not fully cooperated with the PNG police investigation," it said.

"It has complied with all requests for local and expat staff to attend interviews with the police.

"PNG police have interviewed 10 expatriate G4S officers and taken statements from eight of those officers."

Both the PNG police and Australian Federal Police are conducting separate investigations into the riots and Berati's death.