Shipping companies were fined 19 times last year for encroaching into the Poor Knights Islands Marine Reserve off Northland.
Maritime New Zealand said the fines ranged between $2000 and $12,000.
The Poor Knights is a marine reserve above the remains of ancient volcanoes and has been protected since 1981.
Maritime New Zealand said ships longer than 45 metres were banned from the area and they were able to monitor any breaches through a 'virtual fence'.
It said 19 large ships entered the area last year compared with five the year before, probably to save time in travelling to or from Auckland.
Most of the offending vessels were cargo ships and Maritime New Zealand said they were likely trying to save time in travelling to, or from, Auckland.
The area is almost 80km long and runs between the Bay of Islands and Whangarei. It extends up to 35km from the east coast of Northland.
It has been praised for its diving attractions by experts including the late Jacques Cousteau.