Covert agents from the Ministry of Fisheries have caught 27 commercial fish dealers in the South Island illegally buying and selling fish.
Fisheries agents involved in the six-month operation contacted dealers which included fish and chip shops and wet fish retailers from the West Coast to Invercargill, and sold them fish without dockets or sales invoices.
Fish retailers are allowed to sell only fish caught commercially and must have documents to show where they bought it.
Penalties for buying and selling fish illegally include imprisonment up to five years and fines of up to $250,000.
The ministry's Canterbury/Westland field operations manager, Peter Hyde, says black market fish comes either from recreational fishers or from commercial fishers selling their catch outside the quota system.