Families of people who died in the collapsed CTV Building in the 2011 Christchurch earthquake say they will not give up despite a police decision not to prosecute anyone for manslaughter.

Mary Anne Jackson, a receptionist in the building who ran outside when the quake hit, said she was "gutted" by the decision not to prosecute the building owner.

"I think the families won't let it lie," she said. "He might think he's got away with it. He won't be."

Justice Minister Andrew Little said the news was very hard on the families of the 115 victims.

"Right now my thoughts are with the families. A bit like the Pike River families, they will be extremely disappointed that, yet again, a major tragedy, another disaster that seems to have been avoidable - everybody involved in this has walked away scot-free. And that's not right, and I think we have to do better when it comes to accountability for these sorts of tragedies."

He said that could mean a law change in the future to ensure accountability.

"We'd be keen to look at all the information. We do need to be prepared for the possibility that a law change may well be justified."

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Opposition leader Bill English said he would be willing to work with the Government to find a solution to what he described as a "large-scale tragedy" that should be subject to legal liability.

"It does raise the question that if the Royal Commission says the building didn't meet the standards of the time, then you'd expect there to be some liability for that. We'd be happy to work with the Government over whether there's a problem there and how it can be solved.

"This has been through to two independent processes, the Royal Commission and then the court system. and the result looks unsatisfactory. And I'm sure for the families will feel quite unsatisfactory."

Christchurch Central MP Duncan Webb said he really felt for the victims' families - but the decision to prosecute was a complicated one.

"Ultimately that is a decision for the police. It's not a question of politics. My feelings are obviously with the families, as they have to live through that."​

Police Detective Superintendent Peter Read said the decision not to prosecute anyone followed "a very complex, technical investigation involving expert engineering advice, reconstruction and examination of structural elements of the building, excavation of the CTV site, plus legal reviews by the Christchurch Crown Solicitor and Crown Law".

He said the investigation cost $1.18 million, including $1.15m in expert advice, excluding police salary costs.

Up to 13 police at any one time worked on the case and examined over 18,000 documents.

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"The investigation identified significant deficiencies in the building's design, and police considered charges of negligent manslaughter," Read said.

"The Solicitor General's prosecution guidelines were applied, as they are to all prosecution decisions by police.

"However after very careful consideration of all the information, legal advice and expert opinion available to us, Police has concluded that the evidence available is not sufficient to provide a reasonable prospect of conviction in court.

"Furthermore, technical legal obstacles were identified regarding the length of time after the conduct of individuals and the deaths occurring. This is a further barrier to prosecution."

He said police were "acutely aware that there will be disappointment with this decision, particularly for the families and friends of those who died in this tragedy".

"The issues have been very difficult and finely balanced, particularly as the advice and expert opinion has evolved as the investigation progressed," he said.

"Ultimately, the decision must be based on the evidence before us and the thresholds we must meet.

"It is not simply a matter of letting the court decide. Before we put a criminal case before the court we must be satisfied that there is a reasonable prospect of conviction based on the evidence available. Any prosecution must prove its case 'beyond reasonable doubt'. These tests apply to any criminal matter which we put before the court.

"I can assure everyone touched by this tragedy that police has worked tirelessly and diligently on the investigation. I would like to acknowledge the commitment and professionalism of all the staff involved.

"I am aware that some will question why the findings of the Royal Commission of Inquiry into the CTV collapse do not allow a prosecution to proceed.

"While the Royal Commission was a detailed and thorough examination of the issues which ultimately led to the collapse of the building, its evidence and findings are not in themselves sufficient to meet the test for prosecution, even with the further information gathered during our investigation.

"Police intends to meet with families in the near future to discuss the decision further, and answer their questions."

Deputy Solicitor-General Brendan Horsley said police identified potential charges of negligent manslaughter in relation to Dr Alan Reay and David Harding, the engineers responsible for the design of the CTV building.

"Police were assisted by a comprehensive engineering report from Beca, which identified a number of significant deficiencies in the building's design," he said.

"Beca's report was peer reviewed by two independent experts, one of whom is based in San Francisco.

"Negligent manslaughter is an exception to the normal principle that serious criminal offences require a blameworthy state of mind. But 'ordinary' negligence, or failure to take reasonable care, is not enough. The prosecution must prove, beyond reasonable doubt, that the particular defendant's conduct was a 'major departure' from the standard of care expected of a reasonable person in the circumstances.

"This can be a difficult threshold to meet. It requires the jury to be sure that the conduct was so bad that it deserves to be condemned as a serious crime.

"The prosecution must also prove that the defendant's negligence caused the death of the individual or individuals concerned, although it need not be the only cause. This too can be difficult, especially where an extreme event such as a natural disaster intervenes.

"The Crown Solicitor and Deputy Solicitor-General both advised that the 'evidential sufficiency' test was difficult and finely balanced. Both considered there were real issues with proof of 'major departure' and of causation. Neither considered the prospects of a successful prosecution were high.

"With some reservations, the Crown Solicitor thought the evidential sufficiency test could be satisfied, relying heavily on Beca's opinion.

"The Deputy Solicitor-General disagreed, and thought a conservative approach was warranted. While the prosecution case is supported by Beca's report, it is not 'trial by expert' and the guidelines require consideration of other evidence, as well as of likely defences.

"A key difficulty for the prosecution would be in proving the CTV building would not have collapsed in the absence of the identified design errors. The expert peer reviewers were cautious about drawing this conclusion.

"There is also a technical obstacle to prosecution in this case. The Crimes Act requires death to have taken place within a 'year and a day' after the defendants' negligent conduct ceased.

"The 'year and a day' rule has not been the subject of much consideration by the courts. But the Deputy Solicitor-General considered it is likely to be a complete bar to any prosecution. On the most natural interpretation of events, the alleged negligence ceased when the design process was complete, in 1986.

"The year and a day rule is an historical anomaly and law reform is currently being considered."

Nigel Hampton QC, who represents many of the families who lost relatives in the building, said the decision was "sadly disappointing".

"The Canterbury Crown Solicitor thought that the 'evidential sufficiency' test within the Prosecution Guidelines could be satisfied, meaning that, in that experienced prosecutor's view there was sufficient admissible evidence to provide a reasonable prospect of conviction," he said.

"Surely in such circumstances, where a building which contained significant deficiencies in design collapses and 115 people die in that building, a prosecution is required in the public interest."

He said the "year and a day" rule had been applied "somewhat speciously".

"Where a case of manslaughter is brought, relying on an omission to fulfill a legal duty (as I suggest would be the case here), then the 'year and a day' limitation only starts running from the day the omission ceased – and it would be contended here that such omission here ran up to and included February 22, 2011," he said.

"There is no mention at all of the alternative of 'criminal nuisance' which was considered in the Police inquiry into Pike River – indeed the Police there thought that there was a sufficient case to mount such a criminal nuisance prosecution, but chose not to only because WorkSafe/Department of Labour were taking Health and Safety prosecutions.

"But no mention today of that possible charge (under s 145 Crimes Act) here. It is a charge of omitting to discharge any legal duty, such omission being one which was known would endanger the lives, safety or health of the public or any person. Why not?"