The family of an Aboriginal woman who died in police custody in August says they are "devastated" to learn details of her death.

Ms Dhu, 22, died in August after being locked up in the South Hedland Police Station in Western Australia over unpaid fines.

She was taken to the Hedland Health Campus on three occasions.

On her third visit to hospital Ms Dhu was not breathing, did not have a pulse and was unconscious.

The ABC can exclusively reveal the final hours of Ms Dhu's life, including that she was transported from the lockup to hospital in the rear pod of a police vehicle.

In a letter obtained by the ABC the regional medical director explained on August 4, Ms Dhu "was returned to the hospital by the police unconscious, pulseless and not breathing".

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Ms Dhu's grandmother Carol Roe said the information was distressing.

"(She) went to the campus three times, on the third time I don't know if she was dead or alive," Ms Roe said.

Neither the police, the West Australian Government or the Country Health Service would comment on the new evidence.

Ms Roe said her granddaughter should not have been transported to hospital in the rear pod of a police vehicle.

"They knew she was sick, why didn't they ring an ambulance and take her?

"No, they chucked her in the back of a paddy wagon and take her, and that's not right."

Three trips to hospital

Ms Dhu first visited the Hedland Health Campus at 9:20pm on August 2.

According to the letter from the regional medical director, Ms Dhu was seen by a nurse and doctor after complaining of pain from a fractured rib injury that was two months old.

She was sent back to the cells 25 minutes later at 9:45pm "and was subsequently discharged with analgesia (endone) and diazepam to help her settle overnight".

Ms Dhu was brought back the next day just before 5pm, again complaining of a sore rib cage and difficulty sleeping.

After more tests, she was discharged back to police lock up.

On August 4 at 12:45pm, the letter explained that Ms Dhu was again returned to the hospital.

Upon arrival, she was unconscious, did not have a pulse and was not breathing.

"(She) was taken immediately to the resuscitation area where full resuscitation efforts proceeded," the letter outlined.

"Resuscitation was ceased at 1.39pm after almost an hour of intense effort."

The coroner is yet to determine a cause of death.

Pressure builds for urgent inquiry

Ms Dhu's family said an urgent coronial inquiry was needed to ensure all information about her death was released.

West Australian Greens MP Robin Chapple said it was important to piece together Ms Dhu's final hours.

"How long before she got to the hospital did she have no pulse, was she not breathing?" he said.

"Was she in that condition when pulled from the cell? We don't know."

The West Australian Government and police said an investigation into the matter was still ongoing and a report was being prepared for the coroner.

West Australian Labor MP Ben Wyatt said deaths in custody should be on the national agenda.

"Maybe, and this may be the result of the coroner's finding, we need to have another broad review into what is going on," Mr Wyatt said.

Marches in Port Hedland, Geraldton and Perth are being held today as part of a day of action in memory of Ms Dhu and other deaths in custody.