The Ministry said for that reason it was unable to answer further questions about the deaths and investigation.
The referral does not make prosecution inevitable.
Auckland-based legal expert and scholar Dr Bill Hodge said generally, crown solicitors assessed whether prosecution was in the public interest.
He said the solicitors would ask: "Can we win and should we use the resource to this end?"
Marcel Rodriguez Ferrere, an animal law expert, previously said the incident northwest of Auckland was "the biggest mass death" in New Zealand history.
It's understood the dead animals were subsequently dumped on a worm farm.