"The person in charge of the animals took steps to maintain that equipment and the Crown Solicitor advised this provided a defence to potential charges under the Animal Welfare Act."
As such it did not meet the threshold for prosecution as per the Solicitor-General's prosecution guidelines, Orr said.
"The large number of chickens involved, and the highly technical nature of the evidence, required careful consideration, including advice from independent experts."
Orr said systems on the farm were upgraded to ensure a mass death event would not occur again.
Auckland-based legal expert Dr Bill Hodge previously said Crown solicitors would assess if prosecution was in the public interest.
He said the solicitors would ask: "Can we win and should we use the resource to this end?"
Marcelo Rodriguez Ferrere, animal law expert, today said the decision of public agencies not to prosecute was disappointing.
"It seems that their view is, this is just too hard."
The University of Otago senior lecturer said MPI had a responsibility to file proceedings and see whether the chicken company's defence stood up in court.
"This sends a signal to basically anyone in the chicken farming industry that simple failures of power facilities or container facilities won't lead to prosecution."
He said MPI seemed to contradict itself when saying the system was now upgraded so a similar disaster could never happen again, yet the system that failed was "maintained".
Rodriguez Ferrere previously called the incident the biggest agricultural mass death event in New Zealand history.
A director of DB Chicks has been approached for comment.
It's understood the dead chickens were subsequently dumped on a worm farm.
This afternoon, animal welfare group SAFE said the decision not to prosecute showed New Zealand's animal welfare system was broken.
"It is clear now more than ever that MPI should be stripped of its animal welfare responsibilities," SAFE campaigns manager Jessica Chambers said
"This investigation should have been a priority for the Ministry. Instead, they dragged their heels and took over a year to investigate."
Animals Aotearoa, which campaigns for improved chicken welfare, said the decision not to prosecute was sad, but unsurprising.
"When 180,000 birds die and MPI can't act, it really shows how broken the system is and how appallingly low the standards are for chickens bred for meat," executive director Marianne Macdonald said.