Iranian Hamid Kehazaei, 24, was evacuated to Australia after a cut on his foot became infected and developed into septicaemia.
He was transferred to Brisbane's Mater Hospital last Thursday and declared brain dead on Monday.
Kehazaei is on life support and it is understood his family have given the go-ahead for his organs to be donated once it is switched off.
Speaking from the family's home in Tehran, the asylum seeker's mother, Goldone Kehazaei, told Guardian Australia that he was ill and unaware of his surroundings when he boarded the plane.
"I was very upset because if I was there I would have held his hand and given him some comfort," she said.
"On the plane his heart stopped and by the time he got to hospital the infection had spread and blood wasn't getting to his brain.
"Only his heart and lungs were functioning. His brain and kidneys had failed.
"I later found out he was on life support."
The family paid tribute to Kehazaei, describing him as a very sensitive, harmless, lovable person and a gentle soul.
Kehazaei had reportedly complained about unhygienic conditions and a lack of treatment from medical staff as his condition deteriorated.
"Hamid is a victim of the shocking conditions and medical neglect on Manus Island," Refugee Action Coalition spokesman Ian Rintoul said.
"It is inexcusable that he developed septicaemia on Manus Island.
"There are scores of infections on Manus Island and many complaints of the lack of medical attention.
"Asylum seekers on Manus Island are often forced to walk through raw sewage.
"Nothing more obviously reveals Manus Island as an unhygienic hellhole that must be closed."
Candlelit vigils were held in Melbourne, Sydney, Perth, Adelaide, Brisbane and Canberra last night, protesting at Kehazaei's treatment and calling for the Manus offshore processing centre to be closed.