There will be no prosecution after an estimated 180,000 chickens died in a disaster on Auckland's outskirts.

The Ministry for Primary Industries today told the Herald that after taking advice from the Crown Solicitor, it will not pursue prosecution after the broiler farm deaths.

An animal law expert said the decision was disappointing, and sent a signal that authorities would capitulate on prosecuting tough cases in the poultry sector.

The animals died at D B Chicks near Helensville in late November 2019.

"The deaths were caused by an electrical failure of equipment providing ventilation to a large chicken shed," MPI director of compliance Gary Orr said.

"The person in charge of the animals took steps to maintain that equipment and Crown Law 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 D B Chicks has been approached for comment.

It's understood the dead chickens were subsequently dumped on a worm farm.