Please note: This story contains reference to someone who has died.

An Aboriginal man died in custody in New South Wales on Sunday.

The 26-year-old man was an inmate at the Shortland Correctional Centre in Cessnock.

Corrective Services NSW reported the man was found "unresponsive in his cell at 6:50am" and was pronounced dead at 7:00am.

"All deaths in custody are subject to a coronial inquest," they said in a statement.

On Monday, the facility celebrated expansion, opening a new minimum-security wing which hosts 240-beds.

At an opening event, Prison Chaplain and Boandik & Barkindji woman, Reverend Di Langham spoke about the man's death.

"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."

Reverend Langham reflected on the importance of safety and wellbeing for inmates.

"I would like to remind everyone here, as I remind the Aboriginal men in this centre, that these men are our future Elders," she said.

"Whatever they have done and however they have been treated, the way they respond to their time in here, will be the way our future Elders, will be moulded."

With NSW Minster for Corrections Anthony Roberts in attendance, the Reverend acknowledged the high rates of First Nations people dying in custody.

"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."

The man's death is the second reported death in the prison this year. A 37-year-old Aboriginal man died at the facility in April.

By Rachael Knowles