The mother of a teenage boy shot dead by Victoria police has filed a landmark action with the United Nations designed to overhaul Australia’s procedure of investigating the police, Guardian Australia has learned.

Tyler Cassidy was 15 when he was killed in December 2008, making him the youngest person in Australia to be shot dead by police. The handling of the case was broadly criticised at the time, as the primary investigation into his death was undertaken by the police themselves.

Shani Cassidy, Tyler’s mother, has brought the complaint to the United Nations human rights commission, with lawyers arguing that Australia has violated the international covenant on civil and political rights (ICCPR) by neglecting to ensure independent and effective investigation into the circumstances of Tyler’s death.

In a statement seen by Guardian Australia, Cassidy said: “I have made a complaint to this committee because I feel it so important that deaths like Tyler’s be investigated properly by a truly independent body. The investigation was conducted by Victoria police because, in Victoria, there is nobody able to investigate deaths like my son’s. Until such a body exists there can be no justice.”

She added: “It saddens me deeply that, when my son was dying, when he was taking his last breath, there was not one compassionate human being at his side. He might have wanted to say something important, offer his last thoughts. As a mother, to lose a child in this manner is the most horrific of outcomes.

“Tyler’s death was investigated by members of the same police force at whose hands he died. The police officers who killed my son were not even treated as suspects. Contrary to the usual practice of Victoria police when dealing with others involved in homicides, the interviews of the police officers who shot Tyler were not audio- or video-recorded.”

The teenager was killed by a volley of bullets fired by three of the four police officers present at the time of his death. A total of 73 seconds elapsed from the time the leading senior constable approached Tyler to the time he was shot. He was alone at a skatepark in All Nations park in Northcote, Victoria, armed with two knives that he threatened police with. Tyler is known to have been emotionally distressed at the time and had been drinking beforehand.

The subsequent coroner’s inquiry – which relied heavily on evidence prepared by the Victoria police in its investigation into the circumstances of Tyler’s death – concluded that there were a number of deficiencies in the investigation, including delays in contacting police investigators and police breaches of media protocols. The inquiry also found that the police officers involved in the death had acted within the limitations of their training.

Anna Brown, a lawyer acting for Shani Cassidy told Guardian Australia that the case was “emblematic of the structural flaws in the way police-related deaths are investigated in Victoria and other states in Australia”.

Brown, of the Human Rights Law Centre in Melbourne, says the ICCPR, to which the Australian government is a signatory, enshrines the right to life and subsequently the impartial and effective investigation of death at the hands of a state agent. She said action under the ICCPR was a necessary step as Cassidy had exhausted legal avenues in Australia, which, unlike many other democracies has no overarching human rights bill.

A spokesman for the Human Rights Law Centre said: "Rather than simply having review powers, what is required is an independent body with the ability to take carriage and conduct of an entire investigation from the word go.

“Apart from in Queensland, the primary duties in such investigations in Australia lie with the police.”

A spokesman for the attorney general’s office said: “The government does not comment in detail on the content of cases before the UN human rights committee while they are on foot.

“Australia takes its human rights obligations seriously and will make detailed submissions in response to this case, once relevant documents are received.”

A spokesperson for Victoria police said: “We are aware that Ms Cassidy has asked the UN human rights committee to investigate the matter further. If they choose to do so, then we will of course co-operate and provide all appropriate assistance.

“The investigation of this sad event was conducted by the homicide squad on behalf of and at the direction of the coroner. That investigation was oversighted by the Victoria police ethical standards department and was reviewed by the Office of Police Integrity, the organisation charged with oversighting Victoria police. The death and investigation was the subject of an open and transparent coronial inquest.”