Police say no criminal charges will be laid against any individual involved with the management of Pike River Coal before the mine disaster.
Twenty-nine men died after a series of explosions at the West Coast mine that began on 19 November 2010. Police began their investigation the day after the explosion with the aim of assessing criminal liability that could be attributed to any individual, and met with families of the workers in Greymouth on Wednesday night.
Police said there is insufficient evidence to lay manslaughter charges, even though there is ample evidence of widespread departures from accepted standards of mine operations.
They said there is enough evidence to lay a charge of criminal nuisance, but if police did pursue that prosecution it could interfere with ongoing prosecutions brought by the Ministry of Business, Innovation and Employment under the Health and Safety in Employment Act.
Detective Superintendent Peter Read, who led the inquiry team, said if the mine is entered in future new evidence could prompt police to revisit the case. However, he said there is no guarantee that new evidence would lead to a prosecution.
Mr Read said he has spoken to the families about a "very difficult" decision. "I know they will be very disappointed. I can only give them my absolute assurance that we have been meticulous in our investigation and consulted widely as the inquiry progressed."
He said the complex investigation involved interviews with 284 people, 25,000 pages of witness statement transcripts and 34 million pages of documentation relating to the mine's operation. Up to 16 police investigators were involved at any one time and a range of experts provided technical input. Police also sought advice from Crown Law and the Crown Solicitor.
"However at this time, police believe this matter is most appropriately dealt with through the health and safety prosecutions led by the Ministry of Business, Innovation and Employment," Mr Read said.
Grey District mayor Tony Kokshoorn said the West Coast community is disappointed that no criminal charges will be laid, but he is not second guessing the police's decision. Mr Kokshoorn told Radio New Zealand on Wednesday night the case shows the need for the Government to introduce corporate manslaughter laws.
Bernie Monk, a spokesperson for some of the Pike River families, said police and the families are frustrated with the decision.
"They've seen the wrong and everything, but beyond reasonable doubt's basically the main point that shone through on all this. I mean, how can you say beyond reasonable doubt when 29 men are dead? But that's the way our law is set up in New Zealand - and that's the most frustrating bit for them as well."
Mr Monk told Radio New Zealand that the families are discussing legal options they might take to hold somebody accountable.
Pike River Coal Ltd, which went into receivership soon after the disaster, has already been sentenced for health and safety failings that led to the men's deaths.
On 5 July this year, Judge Jane Farish ordered the company to pay reparation to the families and two workers who survived the blast of $110,000 each - a total of $3.41 million.