The 45-year-old was admitted to hospital last week after attempting to take his own life.
He reportedly fell into a deep depression several weeks ago after his asylum application was initially rejected.
A subsequent suicide attempt left him brain dead, with his life support switched off on Sunday.
Refugee advocate Ian Rintoul said the man arrived in Australia in 2014 after being on Nauru since 2012, and leaves behind a wife and four children in Sri Lanka.
"This Tamil asylum seeker is another casualty of deliberately harsh policies to break people who make asylum claims," Mr Rintoul said.
"They are denied natural justice and have been subjected to unacceptable delays, and denied legal help."
Mr Rintoul said the man had been brought to Australia under a Gillard-era arrangement with a group of refugees on Nauru, where they would be brought to the Australian mainland - but their asylum claims would begin from scratch regardless of their progress.
It is understood refugees in that cohort began to have determinations on their claims delivered from 2017.
Mr Rintoul said the delay in processing is resulting in many people who might have been assessed as genuine refugees having their applications denied because of changes in their home countries, with a number of Sri Lankan Tamils having their asylum claims denied recently.
"It's very contentious this question about how much (Sri Lanka) has changed since they left and how safe people are."
The Department of Home Affairs has been contacted for comment.
