Corrective Services NSW have confirmed a 37-year-old man died at Cessnock Correctional Centre on Tuesday morning.
The man was found unresponsive in his cell at about 10am before he was pronounced dead at 11.15am.
In a statement, a spokesperson for Corrective Services NSW said the death was being investigated.
The man's death, the sixth death in custody since the beginning of March, will be subject to a coronial inquiry.
It comes just weeks after the nation marked 30 years since the Royal Commission into Aboriginal Deaths in Custody handed down its recommendations.
It's the fourth death in custody in NSW in the past two months, with a woman aged in her 50s found dead in her cell at Silverwater Women's Prison, and a 35-year-old man's death at Long Bay Hospital, both in early March.
Barkindji man Anzac Sullivan died during a police pursuit in Broken Hill, NSW on March 18.
In Victoria, a man died in Ravenhall Correctional Centre on March 7.
In Western Australia, a 45-year-old man died at Perth's Casuarina Prison on April 3.
