A contract with International Health and Medical Services, which provides healthcare for immigration detainees on Manus Island, will end on Monday.

Amnesty International Australia refugee coordinator Graham Thom feared there was no plan in place to fill the void following the cut-off date.

"The health situation for refugees and asylum seekers in PNG is already dire, but the end of the IHMS contract threatens to turn this into an all-out crisis," Mr Thom said on Friday.

"Despite six deaths, and soaring rates of mental illness and self-harm among this traumatised population, the Australian government seems to have no plan in place for filling the void that will be left by the withdrawal of these services."

However, a spokeswoman for the Department of Home Affairs has confirmed a new health services provider has been engaged to begin work on Tuesday.

"IHMS will work with the new health service provider during a transition period in PNG to ensure service continuity," the Home Affairs spokeswoman said.

"Individuals will continue to have access to appropriate primary health services."

IHMS has extended its contract to provide health services to immigration detainees held in Nauru.

Australian Associated Press