Fishery officers seized 15kg of paua meat and two vehicles during a raid in Wanganui on Wednesday morning.
The operation was the culmination of a six-month investigation into illegal sales of paua and crayfish.
Plain clothes officers visited 15 restaurants and fish-and-chip shops, and found 10 were buying fish outside the Quota Management System.
Several private addresses, shops and restaurants were searched.
Five people are being interviewed and the Ministry of Fisheries says charges will be laid at a later date.