People collecting tuatua from Ripiro Beach could be also taking fully protected toheroa by mistake.
During an influx of tuatua gatherers on the beach south of Dargaville this year, Ministry for Primary Industries (MPI) staff and honorary fisheries officers have found some people with mixed bags of tuatua and toheroa.
"Staff and honorary officers will be using some discretion, but will not tolerate people deliberately or repeatedly taking toheroa," MPI acting district compliance manager Stephen Rudsdale said.
"It's important people know the difference because there's some hefty fines for taking toheroa."
Fisheries officers would be out and about on the beach ensuring gatherers didn't break the rules, he said.
The toheroa fishery was closed across the country in 1982 and it is illegal to harvest, disturb or possess toheroa without a customary permit.
Tuatua beds were far more plentiful and accessible in the area than they had been for many years, Mr Rudsdale said.
"Small toheroa and mature tuatua look very similar and they are sitting together in the same beds. People who are getting a feed of tuatua are picking up small toheroa by mistake."
To help people tell the difference between the two, MPI has printed 5000 fact sheets to distribute among gatherers in the area.
The usually smaller tuatua has a slightly glossy shell, compared with the dull shell of the toheroa.
Another simple test is to sit the shellfish on its base on the sand, with the sharp end standing up. A tuatua should stay standing, balanced on the flat base, while the toheroa, which has a lump on the base of the shell, should fall over.
Toheroa are found only on the west coast beaches of Northland and Horowhenua, and Foveaux Strait beaches in Southland.
The MPI encourages people to report any suspicious fishing activity to 0800 4 POACHER (0800 476 224). All calls are treated as confidential.