A man died at Port Phillip Prison in Melbourne's west on Monday night, Corrections Victoria said.
It is believed he suffered a medical episode. A smoking ceremony was being arranged.
Separately, NSW authorities confirmed a 37-year-old man was found dead in his cell at Cessnock Correctional Centre on Tuesday morning.
Both deaths have been reported to the coroner.
They follow the deaths of five other Aboriginal people in custody across Victoria, NSW and Western Australia since March 2.
A man died at Port Phillip Prison in Melbourne's west on Monday night, Corrections Victoria said
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.