A man has died at Port Phillip Prison in Melbourne's west on Monday night, Corrections Victoria said.
It's 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 coroners in each state.
It follows the deaths 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.
More than 470 Indigenous people have died in detention since a 1991 royal commission report into Aboriginal deaths in custody.