The New South Wales Police Commissioner has rejected a recommendation to develop guidelines for the review of investigations, despite a coroner finding an "inexplicably" deficient police investigation took place after the death of two Indigenous children.

An inquest last year into the deaths of Indigenous cousins, Jacinta Rose 'Cindy' Smith, 15, and Mona Lisa Smith, 16, in December 1987 found their deaths were not adequately investigated by detectives in the NSW country town of Bourke.

State Coroner Teresa O'Sullivan accepted evidence that racial bias played a role in the failings.

"The uncomfortable truth, to my mind, is that had two white teenage girls died in the same circumstances, I cannot conceive of there being such a manifestly deficient police investigation into the circumstances of their deaths," she said.

The two girls' bodies were found beside the wreckage of a Ute on the Mitchell highway, between Bourke and Enngonia, having suffered "non-survivable" injuries.

Non-Indigenous man, Ian Alexander Grant, then 40, was found by witnesses with his arm draped across the body of a bare chested and partially naked Cindy.

Coroner O'Sullivan said: "Horrifyingly, the evidence suggests Mr Grant sexually interfered with Cindy after she passed."

Mr Grant was later acquitted of driving-related offences with a further charge of interfering with a corpse being dropped on the eve of the trial.

He died in 2018.

Coroner O'Sullivan recommended NSW Police Commissioner "develop guidelines for the review of investigations relating to deaths that are the subject of a request for advice from the NSW Attorney General to the Commissioner of the NSW Police Force, where the Attorney General is considering an application for the holding of a fresh or further inquest into the death/s".

She said guidelines should include the methodology of the review; transparency of the review process; the involvement of any experts (including independent experts as required); and consultation with the family of the deceased.

"In formulating the guidelines, the standard operating procedures applicable to the review of homicide investigations should be considered and applied as appropriate," Coroner O'Sullivan said.

Responding to this recommendation, NSW Police Commissioner Karen Webb said it was "not supported".

"I can advise that the existing guidelines utilised by the NSWPF State Crime Command Unsolved Homicide Team already cover the situation for reviewing suspicious deaths," Ms Webs said.

Mona Smith's sister Fiona said the Police Commissioner's response did not surprise her family.

"There's a long-standing history of racism in the NSW Police and it looks like it's going to continue," she said.

The coroner may consider a fresh inquest under Section 83 of the Coroners Act 2009 upon request from the Attorney General if considered appropriate.

Cindy's mother Dawn said at the conclusion of the inquest she wanted to hold someone "accountable" for what they had heard in court, noting the "pain and the hurt are still with us and always will be".

"That will never go away," she said.

"I hope that other families in our position won't ever go through the same thing (that) we did for 36 years."

The recommendation comes in part after a number of requests to hold an inquest into the two girls' deaths was denied as it wouldn't "adduce new or additional information".

In 2018, the National Justice Project, who represented members of the Smith family, lobbied the then NSW attorney-general Mark Speakman to direct an inquest into Mona and Cindy's deaths, based on the limited information then available to the families regarding the girls' deaths.

Then Police Commissioner Michael Fuller sent a letter to Mr Speakman in 2020, arguing the brief of evidence had been subject to "thorough independent reviews in 2004 and in 2018" and recommended not holding an inquest.

"Both reviews found that an adequate investigation was conducted with the available evidence and technology at the time," Mr Fuller said.

"I note that the accused Alexander Ian Grant…was discharged by a jury and is now deceased. All police involved in the original investigation are now discharged and further investigation is not planned by police.

"It is for these reasons that I do not believe an inquest into the deaths of Jacinta and Mona Lisa Smith will adduce new or additional information."

After reading Mr Fuller's letter out in court, counsel assisting the coroner, Peggy Dwyer SC, argued it was "very worrying that that was said".

"The family and community have been thoroughly vindicated in the holding of this inquest," Ms Dwyer said.

In 2022, Mr Speakman said an inquest would not proceed, arguing "a further coronial inquest would not be capable of finding any new information not already known, or of providing any meaningful information or closure for Mona or Cindy's families".

Coroner O'Sullivan disagreed with claims the investigation was adequate.

"Given the deficiencies in the initial police investigation which have not been catalogued or acknowledged until this inquest, the perseverance and strength of the Smith families in pursuing justice in the form of answers and a thorough investigation (albeit many years too late) cannot be overstated," she said earlier this year.

The National Justice Project slammed the Commissioner for rejecting a key recommendation from the coronial inquest, with chief executive and adjunct Professor George Newhouse saying the Coroner sat through many weeks of the inquest and sifted through every piece of evidence to arrive at her findings and recommendations.

"For the Commissioner to now disregard those recommendations in such a cavalier and dismissive way is shameful," he said.

"Commissioner Webb's decision conveniently ignores the criticisms made of her predecessor who misled the Attorney-General about the nature and quality of previous investigations. This is nothing short of a vindication of the "perfunctory and piecemeal reviews conducted by police over the years," which were highlighted by the Coroner in the inquest findings."

Mr Newhouse said the Police Commissioner was "sadly mistaken" to believe the current police guidelines are remotely adequate.

"These guidelines have failed the Smith family, as well as the Bowraville families and so many other First Nations families for half a century. To suggest otherwise is laughable and perpetuates what many people see as the racist culture of the NSW Police," he said.

"Once again, NSW Police let down the Smith family in an appalling fashion and it needs to change. We will be taking this issue up with the Minister for Police and the NSW Parliament,"