Inquiry into conviction of innocent man jailed for 19 years will investigate why crucial witness evidence not given to defence.

In 1986 Alan Hall was wrongly convicted of killing Arthur Easton in Auckland. The crown has acknowledged police interviews with Hall were ‘unfair and oppressive’.

The role of crown lawyers in the prosecution of Alan Hall, a New Zealander who spent 19 years in jail for a murder he did not commit, will be investigated by an independent inquiry, the country’s solicitor-general has announced.

The crown has released the terms of reference for its unprecedented investigation into its involvement in one of New Zealand’s worst miscarriages of justice, as it attempts to understand why crucial witness evidence was not given to the defence. The inquiry will be conducted by Nicolette Levy QC.

It is the first time the crown has turned independent investigators on to itself, after it acknowledged the “unacceptable truth” that an “unanswerable cause of miscarriage” had taken place. But those specialising in wrongful convictions say broader inquiries into the criminal justice system are needed.

Hall, who is autistic and is pākehā – or of European descent – was convicted in 1986 at age 23 of the murder of Arthur Easton in his home in Papakura, Auckland, in 1985. But a description of a man leaving the crime scene as “definitely dark-skinned” was removed from later versions of a witness statement, without the witness being made aware. The crown also acknowledged that police interviews with Hall were “unfair and oppressive”.

In June, the supreme court quashed Hall’s murder conviction. The chief justice, Helen Winkelman, told the court that the departure of the case from accepted standards must “either be the result of extreme incompetence or of a deliberate and wrongful strategy to secure conviction”.

The inquiry will investigate how the failure of disclosure happened; at what points the miscarriage could have been identified by crown lawyers; and identify the actions or omissions of the lawyers involved between 1985 and 2022 in obtaining, and upholding, Hall’s conviction.

“It is critically important that public confidence in the integrity and impartiality of the prosecution process is maintained,” the solicitor-general, Una Jagose QC, said.

Hall’s lawyer, Nick Chisnall, said the terms had been “generously drafted” and would provide a good platform to undertake the inquiry. “They focus not only on the trial but what has happened since, which is really positive and demonstrates a genuine desire to get to the bottom of things.”

But Chisnall, like other defence lawyers and investigators specialising in wrongful convictions, does not believe it will result in a fundamental change to what they say is a deeply flawed criminal justice system. “It is not a system-wide review … we’ve got to be careful about trying to place too much weight on what this can achieve.”

Calls are growing for a royal commission of inquiry, which would also investigate the role of police and the Ministry of Justice, into why the same themes keep appearing in wrongful conviction cases: misidentification, the non-disclosure of evidence, and improperly obtained statements.

“We’re seeing the same series of issues arising again and again,” said Tim McKinnel, an investigator who has worked on some of New Zealand’s most high-profile wrongful conviction cases, including Hall and Teina Pora, who was imprisoned for 21 years after being wrongfully convicted of murder, before his conviction was quashed by the privy council in 2015. “There is a real lack of interest in learning what errors have contributed to a wrongful conviction.”

New Zealand was naive to the risks of miscarriages of justice and driven by a culture of exceptionalism when it comes to trust in its institutions, he said.

The police doggedly pursued confessions, which could lead to false confessions if the suspect had a vulnerability, such as a learning disability, McKinnel added. He said that to make matters worse, criminal defence lawyers were underfunded and under-resourced.

“There’s every reason to think that we have the same issues as other common law jurisdictions, and we have an Indigenous population that’s heavily over-incarcerated – that is reason to think that we may be worse than some other similar jurisdictions.”

One of New Zealand’s leading criminal defence lawyers, Christopher Stevenson, said the known cases represented just the tip of the iceberg. “Possibly 90% of the actual miscarriages, and the factually innocent people wrongly convicted, go unknown.”

But recent cases of wrongful convictions, including those Stevenson has represented, show the same systemic problems cropping up, he said.

“In Canada, when they have had significant miscarriages … they have had a commission of inquiry. They are really powerful processes and incredible lessons were learned from them, and the Canadian criminal justice system dramatically improved as result,” Stevenson said.

“There should be a commission of inquiry looking into the operation of the criminal system [in New Zealand] and why we are having so many cases go wrong.”