A group of commercial fishers in the Thames region are being accused of under-reporting catches and illegally selling snapper to an Auckland fish supply business.
The allegations come after an investigation by the Ministry for Primary Industries (MPI).
Eight people face prosecution after fishery officers searched five locations - including three residential properties across Auckland and Waikato - in December, according to MPI.
Six commercial fishing vessels, a refrigerated truck and $25,000 in cash were seized during the searches.
The group may have supplied more than 20 tonnes of unreported fish to legitimate businesses over the past year, said Andre Espinoza, MPI's team manager of fisheries compliance in the western North Island.
Among the haul was around 800kg of grey mullet and kahawai, along with 230kg of undocumented snapper.
"There are measures in place to ensure the sustainability of our fisheries, including accurate and truthful catch reporting," said Espinoza. "Those who don't respect the rules will be appropriately dealt with under the law."
Espinoza said fishery officers had increased their patrols over the summer period.
Inquiries into the case were ongoing.