Dr Barri Phatarfod from Doctors for Refugees says requests had been made for “at least a year” to the Immigration Department asking for Iranian asylum seeker Hamed Shamshiripour to be moved to Australia.
The man's body was found in bushland near a primary school on Monday, after reportedly not being seen since Saturday.
Manus Provincial Police Commander Chief Inspector David Yapu told SBS World News the death was being investigated.
Hamed was suspected to have a history of mental illness, and refugee advocates and other asylum seekers on Manus Island say they had been calling for extra support to be provided to him.
SBS World News was provided a letter dated August 6, 2016 from Doctor for Refugees that advised immigration officials of Hamed's condition.
"For at least a year we have been writing to the Immigration Department asking for him to be moved to Australia because we knew he was at risk," Dr Phatarfod said.
Behrouz Boochani, an Iranian-Kurdish asylum seeker and journalist, told SBS World News refugees on Manus Island had also sent letters to the Department of Immigration requesting medical treatment for him.
"The refugees wrote a letter about him... that this man has had a mental illness and you have to provide medical treatment for him."
The Iranian asylum seeker is the fifth to die on Manus Island and the seventh in Australian offshore detention.
He was due for resettlement in Papua New Guinea.
“The refugees are very, very angry and sad and we cancelled our protest because we are not going to make any violence and we cancelled the protest and we are going to have a ceremony for him," Mr Boochani said.
Natasha Blucher of the Asylum Seeker Resource Centre felt the Australian government was “failing” refugees.
"The Australian government is failing them, as they have failed this man who died today."
Daniel Webb of the Human Rights Law Centre urged the immediate evacuation of everyone on Manus Island and Nauru.
"The only responsible and humane thing for our government to do is immediately evacuate every single man on Manus, every single family and child on Nauru to safety on Australia.”
Amnesty International is calling for an independent probe into the death.
A spokesperson from the Department of Immigration and Border Protection told SBS World News in a statement it was "aware of a death in Lorengau Township".
"PNG authorities are investigating the matter," the spokesperson said.
"Further questions should be directed to the Government of PNG."
Readers seeking support and information about suicide prevention can contact Lifeline on 13 11 14.
Suicide Call Back Service 1300 659 467.
MensLine Australia 1300 78 99 78.