The head of Australia's Immigration Department has blamed activists and "troublesome outsiders" for a looming and potentially violent standoff over the closure of the refugee processing centre on Manus Island.

Bureaucrats also confirmed Australia will pay up to $250 million a year in services for refugees and asylum seekers in Papua New Guinea even after the facility shuts and Australian personnel leave next week.

And authorities will also have to confront a last-ditch legal challenge - in PNG or Australia - to stop the centre being shuttered on October 31 and returned to the PNG defence force.

Officials confirmed water, food, power and sanitation will be cut off at "some point" next Tuesday, prompting Greens senator Nick McKim to accuse the government of trying to "starve" refugees out of the centre.