Corrective Services NSW and the state's police force are investigating the death of a 26-year-old Aboriginal man at the Shortland (Cessnock) Correctional Centre on the weekend.
"He was found unresponsive in his cell at 6:50am and pronounced deceased around 7:00am," Corrective Services NSW said in a statement.
"All deaths in custody are subject to a coronial inquest."
His death came a day before the jail celebrated the opening of a new minimum-security wing.
"It is with a heavy heart that I share with you that yesterday I was called into Shortland to another Aboriginal death in custody," she said.
"He is a son, a father and a partner in the community. His death will be another traumatic experience for all involved, and I'm including officers and staff."
The Boandik & Barkindji woman prompted those in attendance, including the state Minister for Corrections Anthony Roberts, to address the number of First Nations people incarcerated throughout the country.
"We all know that Aboriginal people only make up 2 per cent of the population on the outside. But in corrections, we make up nearly 38 per cent of the men and much higher numbers in women," she said.
"I urge all of you to keep looking for successful ways to stop our people from actually coming into jails."
A 37-year-old Aboriginal man also died at the prison in April.
There have been 471 First Nations people that have died in custody since the Royal Commission handed down its findings in 1991, the Australian Institute of Criminology reported in May.
Last month, a 27-year-old Gungarri man died after an interaction with Qld Police in Toowoomba.
