The family of an Aboriginal woman who died in police custody have expressed frustration after the police officer who investigated her death on behalf of the coroner was not removed, despite the coroner raising questions over a perceived lack of independence.

Tanya Day, 55, died in hospital on 22 December 2017. She had been in a coma for 17 days, after falling and hitting her head at Castlemaine police station.

The Yorta Yorta woman had been arrested for public drunkenness after falling asleep on a V/Line train from Echuca, and had been taken into custody to “sober up”.

Coroner Caitlin English listed the case for mention on Monday, two weeks before a three-week inquest is scheduled to begin, because she had seen emails from the coronial investigator in the case – Detective Senior Constable Scott Riley – which she said gave an appearance of a lack of independence.

“The concern that this raised to me was with respect to conflict of interest and the appearance that he remain independent,” English said.

In July, Riley emailed the Victoria police civil litigation branch about a request made by the court for the release of a section of the Victoria police manual concerning the investigation of deaths in custody, and asked the lawyers for their “thoughts”. That email was then forwarded to the external law firm that acts for police, with the same request.

Counsel assisting the coroner said they were satisfied Riley had acted independently and had not failed to release any information. Lawyers for Victoria police said there had been “no interference” in the case, and “there would not be, there has not been, there will not be”.

English said she would not remove Riley as the coronial investigator, but would allow him to be cross-examined on this issue at the inquest, and would consider making recommendations around the lack of legislative protection for the independence of coronial investigators. She said it was a concern that police policy was to run the release of police documents past their internal lawyers.

English has already said that she intends to recommend the repeal of the laws.

Day’s family said they were frustrated and disappointed that Riley was not removed as the investigator.

“We are just voicing the concerns of many other families that have been through this, that police shouldn’t be investigating police, and hopefully in the future some recommendations can be made that address this,” Day’s eldest daughter, Belinda Stevens, said outside court.

Barrister Sarala Fitzgerald, who acted for the family on behalf of Human Rights Law Centre, said the email made it appear that internal police lawyers were playing a “filtering” role in what information was released. She said the justification given in court by lawyers for the chief commissioner – that Riley was acting in accordance with police policy and to ensure he did not release anything subject to public interest immunity laws – did not alleviate concerns about a perceived lack of independence.

“The mere fact that it is in accordance with policy doesn’t make it acceptable,” Fitzgerald said.

She said concerns about the ability of police to effectively investigate police in death-in-custody cases had been raised on state, national and international levels.

“Tanya Day died in police custody: police conduct is at the heart of this case,” she said.

Day’s family has argued that the public drunkenness laws, which exist only in Victoria and Queensland, should be abolished.

The inquest, due to begin on 26 August, is the first in Australia to consider whether systemic racism was a factor in a person’s death in custody.