PNG locals employed by security guards at the Manus Island detention centre attacked asylum seekers with machetes, knives and rocks, an interpreter employed by the Immigration Department has claimed.

Azita Bokan alleges asylum seekers used plastic chairs as shields when they were attacked by locals who entered the centre as tensions spilled over on Monday night.

Tents at the detention centre on Manus Island, 2013. Credit:Jason South

She said she saw asylum seekers who had suffered ''massive head injuries'' when locals entered the centre after some detainees had pushed down a fence within the centre.

''Definitely, 100 per cent, I stand by the statement that the local people, including some employed by (security contractor) G4S, they were the ones who caused this drama,'' Ms Bokan told Fairfax Media after flying out of Manus Island on Wednesday.