The 37-year-old man was found unresponsive in a cell at Cessnock Correctional Centre about 10am.

Three ambulance crews were called to the scene, but the man was pronounced dead about 11.15am. The cause of the man's death has not been publicly confirmed.

"Corrective Services NSW and NSW Police are investigating," a Corrective Services NSW spokesperson said. "All deaths in custody are subject to a coronial inquest."

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Separately, on Monday night, a man died at Port Phillip Prison in Melbourne's west, Corrections Victoria said. It is believed he suffered a medical episode.

This week's deaths follow those of five other Aboriginal people in custody across Victoria, NSW and Western Australia since March 2. They include a man aged in his 30s at a NSW prison hospital, and another man and a woman, at Victoria's Ravenhall Correctional Centre and in custody in NSW respectively. Barkindji man Anzac Sullivan, 37, died during a police pursuit in Broken Hill and a 45-year-old man died in hospital in Perth.

NSW Greens MLC David Shoebridge said the deaths were a national crisis and reinforced the urgent need for the 339 recommendations of the Royal Commission into Deaths in Custody to be implemented.

"Every First Nations death in custody is an inevitable result of the racist criminal justice system that results in First Nations people in NSW being the most incarcerated people in the world," he said.

More than 470 Indigenous people have died in detention since a 1991 royal commission report into Aboriginal deaths in custody.

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