Kumanjayi Dempsey, 44, died at the Tennant Creek Watch House in the Northern Territory on December 27 after being found unresponsive in her cell.
The Aboriginal woman had been arrested and charged with aggravated assault after an incident on Christmas Day.
A post-mortem was conducted on Friday, listing her cause of death as undetermined, with a pathologist to conduct toxicology tests before a formal cause of death can be published.
Assistant Commissioner Peter Malley said any death in custody was thoroughly investigated by Crime Command detectives with oversight from the Professional Standards Command.
"While further testing is required to determine a formal cause of death, the current information suggests that the woman suffered a medical event alone in her cell," he said in a statement on Monday.
Police had reviewed CCTV footage in which she was seen to fall to the ground.
The circumstances surrounding the incident, from policy and procedures governing watch house care to the medical history of the deceased will form part of a brief of evidence to the coroner, Mr Malley said.
"I have full confidence in the independent processes that already govern how police investigate death in custody matters."
Mr Malley said he extended his condolences to Kumanjayi Dempsey's family and friends.
She is understood to have suffered rheumatic heart disease but no notice of that was given when she was taken into custody, police said.
At watch houses in Darwin, Katherine, Palmerston and Alice Springs a custody nurse checks detainees' medical records and conducts health checks, but there is no nurse at the Tennant Creek watch house.
O'Brien Criminal & Civil Solicitors, who are acting for the family, said her death was the fourth Aboriginal death in custody in the NT in 2025.
It was a "devastating indictment of systemic failure", 34 years after the 1991 Royal Commission into Aboriginal Deaths in Custody, the law firm said in a statement.
"We are concerned about the ongoing inadequacy of the conditions of cells in Tennant Creek Watch House, and Northern Territory detention facilities more broadly."
Kumanjayi Dempsey's death followed the recent refusal by the NT government to allow the UN Working Group on Arbitrary Detention to inspect NT detention centres, including watch houses.
That was a breach of Australia's international human rights obligations under the Optional Protocol to the Convention Against Torture, the law firm said.
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