Papua New Guinean police are investigating the alleged murder of a security guard at the Manus Island detention centre.

The deceased man was beaten and drowned, and his body discovered at about 11pm on Friday, Manus Island MP Ron Knight said.

Manus Island provincial police commander Alex N’Drasal said the victim’s name was Roger Elvis and he was a Wilson Security employee, aged about 25 years old.

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The Immigration Department says it’s aware of the death of a locally-engaged subcontractor but says the allegations of murder are a matter for the PNG police.

The department understands police are investigating.

Chief inspector N’Drasal told AAP there were five suspects and police had apprehended two of them.

Those involved were under the influence of a local home brew drink in similar strength to moonshine, he said.

“(The victim) was thrown into a river,” he said.

The inspector said in another incident at midnight on Saturday morning another detention centre worker had his hand cut off in a machete attack.

Chief inspector N’Drasal said the man was wearing his work uniform at the time of the attack but could not confirm his employer at the centre. The man is receiving medical treatment in hospital.

“His hand is in the morgue,” he said.

Mr Knight said there were also investigations under way into separate claims that three Australian expat workers at the centre were involved in the gang rape of a local woman.

The department says it has not been notified of any allegation of sexual assault and has no information about a worker’s hand being severed.

The Australian Greens are calling for the government to explain the “brutal attacks” at the centre.

“What on earth is going on inside Manus Island?” Greens Senator Sarah Hanson-Young said on Monday.

“Reports of rape, murder and machete attacks paint a picture of Manus in meltdown.”

Senator Hanson-Young also urged contractors Wilson Security and Transfield Services to be clear about what they knew.

Comment has been sought from Wilson Security and Transfield Services.

-AAP