"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," Inspector Symonds said.
The Ministry of Primary Industries is also considering action against the men.
Kaeo's Constable Richard Avery said theft was also a problem in Whangaroa Harbour, where the livelihoods of several 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 suspects' vehicle.
The quantity of oysters taken, one bin full, was relatively small, so the offenders were released with a pre-charge warning. The oysters were returned to the owner.
The men claimed it was their right to take farmed oysters, but it was theft, Constable Avery said, adding that anyone was allowed to collect wild oysters, to a limit of 250 per day.
Inspector Symonds said police would have a boat on the water in the Bay of Islands this summer, and would work closely with the Ministry for Primary Industries. As well as watching for theft and fishing violations, they would be making sure boaties were behaving responsibly.
While there was no alcohol limit for skippers, police had the power to intervene if they believed the person in charge of a boat was not competent, due to drink or other causes.
"We're not the party police there to stop you having fun, and we're not saying don't drink. Just do it responsibly, and make sure you have a way of getting home safely," he said.