Police had been set to charge the former lead psychiatrist at Lake Alice psychiatric hospital child and adolescent unit with ill-treating former patients by subjecting them to electric shocks as punishment.
But Dr Selwyn Leeks was not interviewed during the most recent police investigation into what happened at the unit in the 1970s, and he was deemed too ill to face trial.
Leeks died in early 2022, aged 92. He had lived in Australia for more than 40 years and continued to work until 2006, when he gave up his medical licence ahead of a professional hearing into his conduct.
Charges were laid against former nurse John Corkran, known as Dempsey Corkran, the only former Lake Alice staff member to have faced prosecution.
Read more: Where's the accountability?' Anguish for Lake Alice survivors, campaigners over Lake Alice ruling
Corkran was accused of injecting children with the painful paralysing drug paraldehyde, but the High Court this year ordered a permanent stay on the prosecution because of the 91-year-old's poor health.
The decision left former patients unhappy, as no one had been convicted for what happened at the Rangitīkei institution.
Police sought to interview another former nurse about similar allegations, but they were also too unwell to be spoken to.
A further nurse was interviewed. It was decided there wasn't enough evidence to prosecute.
Information about the latest police investigation was released to RNZ under the Official Information Act.
Previous police inquiries into the Lake Alice unit the 1970s and early 2000s did not result in charges being laid.
At the Royal Commission of Inquiry into Abuse in State Care's look into the Lake Alice unit in 2021, police apologised for shortcomings in the early 2000s probe.
During the Royal Commission hearings, former patients, including many who did not have a psychiatric illness, recalled horrific instances of physical and sexual abuse.
And, former patients Paul Zentveld - with the assistance of watchdog group the Citizens Commission on Human Rights - and Malcolm Richards have taken cases to the United Nations Committee Against Torture.
It ruled the pair were tortured.
Officers began looking over the Lake Alice file and making inquiries from 2018. This briefly paused when the UN released its Zentveld findings in late 2019. Its Richards findings were released last year.
"In February 2020, as a consequence of the [UN] report, a full criminal investigation was commenced into the actions of staff at the unit."
The police investigation, and its contact with Leeks and other staff
Police interviewed 63 former patients on video. Nineteen declined an interview, but agreed previous statements during a civil claim in the late 1990s and early 2000s could be used. Twenty declined to speak and 31 had died.
Thirteen former staff members were interviewed, while 39 had died, although some had made past statements that were considered.
The police information released to RNZ said that in 2021 officers contacted Leeks' wife "with a view to interviewing him and putting the allegations to him".
Leeks' wife said he was unwell. She authorised police to contact his medical practitioners. That happened and a neuropsychiatrist prepared a report into him. A second medical opinion was sought, but could not take place for Covid-19 reasons.
"Based on the neurological assessment it was decided Dr Leeks was, firstly, unfit to be interviewed and, secondly, unfit to stand trial."
If Leeks was ruled fit to stand trial, he would have faced charges of wilfully ill-treating children by using electric shock therapy as a form of punishment, and police would have sought his extradition to New Zealand.
"The exact number of charges or the names of various patients those charges would relate to were never finalised," police said.
Corkran declined to be interviewed by police.
He first appeared in court in late 2021, facing eight charges of ill-treating children, between 1974 and 1977.
Court documents alleged Corkran injected boys with paraldehyde for reasons including them running away; calling him a bastard; "being smart"; and because a boy was "enjoying himself too much, laughing and having jokes with friends".
Six alleged victims were named in court documents, while another charge referred to an unidentified child.
The ill-treatment charges had a maximum penalty of 10 years' jail.
Police also approached a second nurse about allegations they had injected children with paraldehyde, but they were also too ill to face prosecution.
That nurse, who was not named, lived overseas.
"Information was obtained from the individual's partner and from the manager of the advanced dementia care facility where the individual was resident at the time," police said.
"As a result they were not interviewed, and a decision was made that no criminal prosecution would commence."
Another nurse, also not named, was interviewed about allegations they had misused paraldehyde.
The former nurse was asked by "specially trained suspect interviewers" about allegations against them, and previous statements they had given.
"Where documentary evidence supporting an allegation was available, for example a nursing note, the interviewee was asked to look at the document in question and confirm it was their handwriting/initials on the document.
"The interviewee was then asked to provide further information about the incident and anything else the interviewee could remember about it."
After the interview, police analysed the evidence they had and decided there was not enough to lay charges.
The latest police investigation found electric shocks, sometimes without anaesthetic, were forced on 51 patients on 140 separate occasions as punishment.
On all but one occasion it was alleged Leeks was responsible. On the other occasion it was alleged a nurse, who is now dead, was responsible.
It also found 68 patients were administered paraldehyde on 350 separate occasions as punishment for running away, misbehaving at school, fighting or being cheeky, among other reasons.
"The strength of the evidence lay in the sheer number of patients describing similar types of events where, in many cases, they had not spoken to other patients since being with them at Lake Alice."
No charges were laid over allegations of sexual abuse because the alleged offenders were dead, there was not enough evidence, or the alleged offenders could not be identified. Specifically, the police looked into three allegations of rape and three of sodomy.
New details revealed should prompt soul searching within range of New Zealand authorities- human rights organisation
Citizens Commission for Human Rights director Mike Ferriss said the released police information showed that if Zentveld and the commission had not taken the matter to the UN Committee Against Torture a new investigation would not have happened.
"Now at long last there has been a proper criminal investigation," he told RNZ.
"Even though almost 50 years have passed, the police were able to identify over 140 separate occasions involving 51 child patients where electroshocks were used as punishment by the psychiatrist, Dr Leeks. The shocks were on their heads, arms, legs and even, on 13 children, their genitals."
That, along with the use of paraldehye, was "an alarming number of torturous assaults on children, and it only represents a portion of what happened there".
"The children - now adults - always knew what happened to them was criminal and this vindicates them and their life-long struggle for accountability.
"There should be a lot of soul searching by the authorities who covered this up, from the Medical Council, who refused to investigate complaints, to the Royal Australian and New Zealand College of Psychiatrists, and the Nursing Council, who did nothing - not to mention Crown Law, who sat on the evidence for three decades," Ferriss said.