Disturbing claims have emerged over the circumstances of an asylum seeker's death at Sydney's Villawood detention centre.

It is understood the 26-year-old Afghan man, Ali Ahmad Jafari, died of a heart attack.

Fellow detainees say guards were slow to come to his aid and it took more than an hour to call an ambulance.

Furthermore, refugee advocates say Mr Jafari was only in detention because of a bureaucratic mistake.

Last night, he became the 14th asylum seeker to die in Australia's detention system in the past five years.

Ali Madadrazaie met Mr Jafari in detention about eight months ago and described him as "the greatest guy".

He told the ABC's PM program Mr Jafari's room-mate raised the alarm after Mr Jafari returned from the gym and complained of pains in his chest.

"He went to call the officers, c'mon guys, this guy is in so much pain, c'mon do something for him, and they just said to him like, 'oh, I think you guys are joking, yeah'," he said.

"They just took like four, five - more than five minutes... after then when the officers came look at him, and they called the supervisor.

"The supervisor came then, they make sure after that they call the nurses. His pain was probably from 6pm, and [it was] 7:30pm when he get to ambulance."

Villawood detainees found out Mr Jafari had died in hospital when police arrived at the detention centre around 9pm on Thursday night.

Police determined there were no suspicious circumstances and are now preparing a report for the coroner.

Mr Madadrazaie and refugee advocates who knew Mr Jafari say he had appeared fit and healthy, though anguished about his position in detention.

But - according to Mr Madadrazaie - he had problems sleeping and took heavy doses of sleeping tablets.

"He was in the gym for 15-20 minutes, every day he always doing gym like 15-20 minutes to get a bit tired to go to bed, get sleep, cause he couldn't get sleep and he was just using sleeping tablets all the time," he told PM.

Refugee advocates argue Mr Jafari, a member of Afghanistan's Hazara minority, should not even have been in detention.

They say he spent periods of his life fleeing danger.

His mother took him and his siblings to Quetta in Pakistan when he was 10 after his father was suspected to have been killed by the Taliban.

He later sought asylum in the UK but was deported back to Afghanistan in 2009, apparently because his grounds for protection were deemed to be insufficient.

Then he fled to Australia and arrived on Christmas Island in 2010, saying he had been beaten by the Taliban.

His claim for asylum was rejected and he appealed the decision.

He had spent most of the past three years in detention, but according to refugee advocates, the Immigration Department allowed Mr Jafari to live in the community in February 2012.

But seven months later he was detained again at Villawood.

Ian Rintoul, spokesman for the Refugee Action Coalition, told PM that was based on a bureaucratic mistake.

"He was brought back into detention in September 2012, supposedly because of a conviction for some offence in Britain," he said.

"But that turned out to be false and he got a police clearance in May this year, but Immigration certainly hadn't acted on that clearance."

The ABC asked the Immigration Department to respond to the claims about Mr Jafari's death and detention, but it declined.