A marine reserve off Northland has become plagued with ships - mainly overseas cargo ships - illegally crossing into its waters, marine law enforcement says.
A total of 19 infringement notices were handed out last year for illegally entering the Poor Knights Islands Marine Reserve, up from five in 2016, Maritime NZ says.
The almost 80km-long reserve area between the Bay of Islands and Whangarei is often encroached on by overseas cargo ships who navigate through the reserve to save time and cost, Maritime NZ investigator Nick Dowden says.
"We want ship operators, masters and navigators to understand that this area is protected for a reason, and we take transgressions against the environment very seriously."
The area is one of two that ships are told to avoid in New Zealand - the other being the Three Kings Islands Marine Reserve - but ships are more likely to enter this reserve according to Maritime NZ data.
A digital geo-fence alerts Maritime NZ to ships entering the reserve and crews are instructed to leave the area immediately.
The operator of the vessel can be fined up to $12,000 and the master $2000.