A Papua New Guinean security guard at the Manus Island detention centre was allegedly murdered over the weekend and another had his hand chopped off in a machete attack.

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

Manus Island provincial police commander Alex N'Drasal said the victim was a Wilson Security employee, aged about 25-years-old.

There are five suspects in the murder and police have apprehended two of them so far, he said.

Those involved were under the influence of a local home brew drink, he said.

"(The victim) was thrown into a river," Commander N'Drasal told AAP.

He said in a separate incident at midnight on Saturday morning, another detention centre worker, who had his uniform on at the time, had his hand cut off in a machete attack.

Commander N'Drasal said he did not know who the man's employer was.

The man is receiving treatment in hospital.

"His hand is in the morgue," he said.

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

Comment has been sought from Transfield, Wilson Security.

The Department of Immigration and Border Protection said it is aware of the death of a locally engaged subcontractor, and understands PNG Police are investigating the circumstances.

But the department has not been notified of any allegation of a sexual assault, and has no information that the hand of a Regional Processing Centre worker had been severed.

"Allegations of this nature are a matter for the Papua New Guinea Police Force," a spokesperson told AAP.