"It is theft and you will be prosecuted if you are caught. You can also expect us to seek forfeiture of your boat, your car, and anything else used to carry out the crime."
The Ministry of Primary Industries is also considering action against the men. Kaeo police Constable Richard Avery said oyster thefts were a problem in Whangaroa Harbour, where families depended on oyster farming.
Two people had been arrested last month after they were spotted raiding an oyster farm. The owner followed them by boat and police were able to stop the vehicle.
The amount of oysters taken was small so the offenders were given a pre-charge warning and the oysters returned.
The men claimed it was their right to take farmed oysters but it was theft, Mr Avery said.
Anyone is allowed to collect wild oysters to a limit of 250 per day.
Mr Symonds said police would have a boat in the Bay of Islands this summer and work closely with MPI.
While they will be watching for theft and fishing violations, they will also make sure boaties are behaving responsibly.