The concession was one of a slew of shortcomings released today in a review into the investigation of the 1970 murders of Harvey and Jeannette Crewe at their Pukekawa farm.
Police also acknowledged failures from dealing with evidence at the investigation's outset to Commissioner Mike Bush's comments at Detective Inspector Bruce Hutton's funeral last year.
One of the key findings in the 330-page report was that there was "a distinct possibility" the brass .22 cartridge case used to implicate Thomas may have been planted and if so, it was likely police were responsible.
Arthur Allan Thomas was found guilty of the murders in 1971 and again at a retrial in 1973. But in 1979, after he had spent nine years in prison, he was granted a pardon on the basis that the police case against him was not proved beyond reasonable doubt.
He was paid $950,000 compensation.
In 2010, the Crewes' only child, Rochelle, asked police to reopen their homicide investigation in a bid to find out who killed her parents.
Rochelle was 18 months old when they died. She was found crying in her cot five days after they were last seen alive.
While there was no new evidence implicating any person as being responsible for the crime, the report cleared Lenard Demler – Rochelle's grandfather – who had been a significant suspect when the enquiry began.
Police also said there was no credible evidence to suggest Jeannette Crewe's sister Heather Souter or local farmer John Eyre had anything to do with the murders.
Despite police acknowledging there might have been corruption used to have Thomas convicted, they stood by the opinion of Solicitor-General Paul Neazor who said in 1981 that there was not enough evidence to support a prosecution against any member of police.
However, David Jones QC released his review of the police report today and he disagreed.
He said there was enough evidence to lay charges against Hutton, the officer who headed the case.
The cartridge case in the garden was a "clear finding of fabrication", he said.
Though today's report could not pin the blame on anyone, police said the killer was someone who had access to items from the farm, namely the wire found around Harvey Crewe's body and the axle that had been previously fitted to Thomas' trailer.
Police still believed Thomas' firearm was most likely to have fire the fatal bullets.
Acting Deputy Commissioner Grant Nicholls apologised to Rochelle Crewe over the case, which Jones said would be perennially unsolved unless significant evidence emerged.
"The report shows some aspects of the original investigation were done well but there were shortfalls that led to missed investigative opportunities which have left her with enduring uncertainty over the death of her parents . . . I've also apologised over the report's finding that police could have reviewed the investigation into her parents' murder sooner," he said.
Commissioner Mike Bush caused controversy in April last year when he spoke at Hutton's funeral and praised his work.
"As a result of the eulogy comments at the funeral of Mr Bruce Hutton, I stepped aside from the review process to avoid any possible perception of a conflict of interest . . . The review findings add to my deep sense of regret at having agreed to speak at the funeral," he said.
The review into the case has cost $400,000 to date and amassed more than 92,000 pages of work.
In a video statement on the Police website, Mr Bush said the review was undertaken to "provide answers to Rochelle Crewe about the death of her parents 44 years ago''.
"Because of the passage of time, we unfortunately aren't able to provide all of the answers to these enduring questions. But thanks to the review team's work, we now have the best understanding possible of this case.
"This has prompted us to apologise to Rochelle for the shortfalls in the original investigation, and for the anguish that has caused.
"The review represents a huge amount of detailed, meticulous, thorough investigation.
"I'd like to thank Detective Superintendent Andy Lovelock and his team for their dedication and hard work. I hope the review provides Rochelle and her family with peace of mind for the future. I sincerely wish them well.''
In a statement, Mr Bush said: "Deputy Commissioner Grant Nicholls has extended an apology to Rochelle on behalf of New Zealand Police for a number of identified shortfalls in the original investigation which led to missed investigative opportunities and allowed for continuing public speculation and commentary over the years.
"Police deeply regret the anguish this has caused Rochelle and family members. The apology also encompassed the fact that Police did not conduct an assessment and review of the original investigation either after Mr Arthur Thomas was granted a free pardon or after the Royal Commission of Inquiry released its findings.
"As a result of my eulogy comments at the funeral of Mr Bruce Hutton, I stepped aside from the review process to avoid any possible perception of a conflict of interest. I have already made a public apology for these comments and I reiterate that apology today. The review findings add to my deep sense of regret at having agreed to speak at the funeral.''
- By Rob Kidd of APNZ
