Remediation NZ stockpiled 20,000 tonnes of contaminated waste at its Uruti site.
The incident occured at the Tui Oil Field off the coast of Taranaki.
Although some avocados are already grown in the region the number is small compared to places like Bay of Plenty or Northland.
Among the dead fish were three at-risk species.
A 21-year-old man has been charged with a double-murder.
The council released around 200 Honshu white admiral pupae.
The 21-year-old man charged with the murders has been remanded in custody.
The bodies were discovered at a Hāwera property early on Monday morning.
One person has been arrested.