An Amnesty International report has described gross human rights abuses on Manus Island, including overcrowding and inappropriate treatment of mentally ill patients.

In the report This Is Still Breaking People, Amnesty claims the living conditions on Manus Island breach international standards, with asylum seekers living in cramped and dirty conditions and expected to sleep on thin mattresses on the floor. In the most populated compound, 414 asylum seekers live in close quarters, the report says.

The report also reveals the use of a small, makeshift, separate compound where mentally ill patients are guarded, there is no air flow and asylum seekers must be escorted to the toilet. The mental health team leader for health services provider International Health and Medical Services (IHMS), Gareth Lee, says in the report there was no full-time psychiatrist at the centre.

A spokesman for Immigration Minister Scott Morrison said the mentally ill asylum seekers referred to in the report were accommodated in single rooms.

The report comes as the government has passed amendments to the Migration Act that will leave asylum seekers in indefinite detention if they receive an adverse security assessment by ASIO.