An asylum seeker being held at an Australian detention camp on Manus Island has been found dead, hanging from a tree.

Hamed Shamshiripour, a 31-year-old Iranian national, was found near the East Lorengau refugee transit centre on Papua New Guinea's Manus Island on Monday.

The Australian immigration department said it was aware of the death and the Papua New Guinea authorities are investigating the matter.

The circumstances surrounding Shamshiripour's death are unclear.

But refugees on Manus are questioning initial reports that he had committed suicide after injuries were reportedly found on his body.

READ MORE: Why Australia is paying a $53m asylum seeker settlement

Shamshiripour's death has sparked outrage by other refugees and human rights groups who blame officials for failing to treat him despite a long history of mental illness.

Behrouz Boochani, a journalist and Iranian refugee who has been held on Manus Island since 2013, said on Twitter that eight months ago, refugees had sent a letter to authorities "stating [Shamshiripour] needs medical treatment".

He added that authorities "did not care".

There are currently more than 790 asylum seekers are being held on Manus Island, according to government data from July.

Under Australian law, anyone intercepted while trying to reach the country by boat is sent to detention centres on Manus Island or the Pacific island of Nauru for processing.

The Australian government maintains they will never be eligible to be resettled in Australia.

'Absolute tragedy'

Rights groups, as well as the United Nations, have condemned the prison centres as inhumane.

Kon Karapanagiotidis, the chief executive of the Melbourne-based Asylum Seeker Resource Centre, said Shamshiripour "would be alive today if an Oz hospital [got him] the urgent care he needed".

Daniel Webb, the director of legal advocacy at the Human Rights Law Centre, called the asylum seeker's death an "absolute tragedy".

"This man deserved to be treated with basic decency and respect ... Instead, [the Australian] government chose to be cruel, imprisoning him indefinitely on a remote island in truly awful conditions," he said in a statement.

#Manus Island



5 refugees now dead due to Oz Gov neglect



Australian run



1,671 days



3 billion $ spent



850 imprisoned



0 safely settled pic.twitter.com/pTQBUqoFd7 — Kon Karapanagiotidis (@Kon__K) August 7, 2017

Shamshiripour is the fifth man held on Manus Island, and the seventh refugee in total, that has died while detained in offshore processing centres since the Australian government began the policy in 2013.

A Papua New Guinea court ruled last year that the detention centre on Manus was unconstitutional, and Australia is set to close it by the end of October.

Many at the detention prison have refused to leave the camp and go to transit centres such as the facility where Shamshiripour was being held out of fear the local population will attack them.

'Evacuate all'

Refugees at the Manus Island prison camp had been protesting their treatment earlier this week following the leak of phone transcripts of a conversation between Australian Prime Minister Malcolm Turnbull and US President Donald Trump.

The two leaders discussed a refugee deal between Australia and Trump's predecessor Barack Obama that would resettle up to 1,250 asylum seekers held in offshore prisons.

READ MORE: Offshore and off limits: Australia's refugee centres

Trump is heard telling Australian prime minister that the agreement is "stupid", while Turnbull calls on the US president to "respect" the deal, but implies that the agreement does not mean the US would have to take any refugees, only vet them.

Following Shamshiripour's death, rights groups called on the Australian government to "immediately evacuate" those being held on Manus and Nauru.

"Our government cannot guarantee their safety or that a single person will be resettled in America," the HRLC's Webb said.

"After four years of fear and violence and shootings and death, the only humane and responsible way forward is to immediately evacuate every single man, woman and child held on Nauru to Manus to safety.

"Otherwise, further tragedy is inevitable."