There was no nurse on duty at the Tennant Creek watch house when an Indigenous woman died in custody there on Saturday, Northern Territory Police have admitted.

NT Police say they believe the death in custody of the 44-year-old was the result of a "medical episode", while acknowledging there was no nurse on duty to check her records.

The ABC reports Police say officers were conducting cell checks at the watch house at about 1pm when they found the woman - a mother of five - unconscious, and she was rushed to hospital but could not be revived.

Acting Assistant Commissioner Peter Malley told media on Sunday that the woman underwent a "custody health assessment" after her arrest for an alleged aggravated assault at about 6pm on Thursday.

Acting Assistant Commissioner Malley said officers conducted a cell check at 12.30pm and it was at the next scheduled check that she was found unresponsive.

"At one stage she was in a cell with another lady, there was no interactions between the two and the other lady was released from custody 20 minutes before the episode occurred," he said, the ABC reports.

"We reviewed the CCTV and at 12.34 she was in the cell alone and was seen to fall to the ground, so at this stage we believe it's a medical episode, but we won't have a post-mortem conducted until later on in the week."

Acting Assistant Commissioner Malley said, unlike in Darwin and Mparntwe/Alice Springs, the Tennant Creek police station did not have a custody nurse on duty who could have checked the woman's medical records.

"For a normal arrest, a custody incident, we don't do that," he said.

"The police officer that accepted her into custody conducted that [assessment] and there was no disclosures, no visible injuries to the female at that stage and so everything was believed to be OK.

He said the woman's cell was "still currently locked down" as a crime scene and would remain so until after the post-mortem was conducted.

Acting Assistant Commissioner Malley said the investigation was being led by the Major Crime Unit with oversight from the force's Professional Standards Command, and that he expected the results of the post-mortem to be available by the end of this week.

'Aboriginal people continue to be locked in spaces that are fundamentally unsafe'

The National Network of Incarcerated and Formerly Incarcerated Women and Girls expressed outrage at the death.

"This is not an isolated incident. It is not a 'tragic event'. It is not bad luck or bad timing. It is the predictable outcome of a system that continues to scoop Aboriginal people off the streets, cage them, neglect them, and kill them," said Network spokesperson Debbie Kilroy

"The Northern Territory has a long and brutal history of Aboriginal deaths in custody. Time and again, we are told the cause of death is 'yet to be determined'. Time and again, families are left waiting for answers that rarely bring accountability, justice, or change."

Tabitha Lean, also of the National Network, said Aboriginal women are being criminalised due to racism, poverty, homelessness, mental distress, and survival.

"Watch houses, designed for short-term holding, have become sites of prolonged detention, neglect, and lethal harm. Police cells are not health facilities. Police are not care providers. And yet Aboriginal people continue to be locked in spaces that are fundamentally unsafe," she said.

The National Network urged "Immediate transparency and independent investigation into this death; full accountability for all officers and systems involved; an end to the use of watch houses as de facto detention facilities; the urgent decarceration of Aboriginal people, particularly women; and investment in Aboriginal-led, community-controlled responses to crisis, health, and safety".

"How many more names must be added to the list? How many more families must receive a phone call that shatters their lives? How many more Aboriginal people must die before this country admits that policing and imprisonment are killing us? This must stop," the Network said in a statement.

More than 30 years on from the 1991 Royal Commission into Aboriginal Deaths in Custody, First Nations deaths in custody are now at their highest level since reliable national record-keeping began in 1979/80.