Immigration officials said they were helping a police investigation into the "sudden death of a 36-year-old detainee" at Villawood detention centre on Sydney's western fringe.
"The Fijian national had been detained at (Villawood) since August 17," the department said in a statement.
A spokesman said the "circumstances surrounding his death are not yet clear" and described it as a "tragic incident".
Refugee advocates who have spoken by phone to inmates said the man, who they say was due to be deported Monday, had "thrown himself from the rooftop" of a building inside the 300-person facility, and was believed to have died instantly.
"The other detainees are very distressed," said activist Jamal Daoud.
A handful of Tamil detainees took to the roof in protest after the incident, with a larger group gathering in the grass below with bedsheets folded into the letters "SOS", local television showed.
"They are very fearful that they will be deported," Tamil community spokeswoman Sara Nat