New South Wales has suffered its first Indigenous death in a police cell in 16 years, the Aboriginal Legal Service says.

The ABC reported on the death of Rebecca Maher, 36, at the Maitland Police Station on July 19, but police did not reveal at the time that she was Aboriginal.

Police said she was found dead in a cell less than six hours after she was arrested walking along a road in Cessnock while appearing intoxicated.

The Aboriginal Legal Service's chief executive, Gary Oliver, has accused police of failing to follow proper protocols, which dictate officers must alert the Custody Notification Service (CNS) whenever an Indigenous person is arrested.

"There wasn't any notification that Ms Maher was being held by police," Mr Oliver said.

"We're very concerned that there's been a procedural failure this time.

"If the CNS had been used by police when they detained Ms Maher, there may have been a different outcome."

He also criticised police for not notifying the Aboriginal Legal Service of Ms Maher's death until August 12.

The ABC has sought comment from NSW Police about whether officers were aware Ms Maher was Aboriginal, and why they did not alert the custody notification service of her arrest.

"A critical incident investigation is underway with all information to be provided to the coroner. It would be inappropriate to comment further," a police spokesperson said.

Mr Oliver said the custody notification service ensured Aboriginal people who were arrested have access to a lawyer and a health check.

"Even if a person is seen to be intoxicated, the police still ring us and let us know they've got a person in custody, and NSW Police ensure that person in custody is made safe," he said.

Mr Oliver said it was the first Indigenous death in a police cell since the notification service was introduced in 2000.

Police launched a critical incident investigation immediately following the woman's death, which will also be examined by a coronial inquest.

The custody notification service was one of the key recommendations of the 1991 Royal Commission into Aboriginal Deaths in Custody.

The notification service was recently thrown a lifeline after a funding stoush between the Federal and State governments.