The national grid operator Transpower is appealing to boaties to keep well away from the Cook Strait cable.
It's reminding them that fishing and diving is prohibited in a zone either side of the cable, which provides up to 40% of the North Island's electricity load.
Transpower's warning follows the conviction of a man whose runabout drifted into the exclusion zone while he was fishing and a colleague was diving for crayfish.
He was fined $2000 and ordered to pay court costs of $130, but could have been penalised anything up to $250,000.
While the cable was unharmed in the incident, Transpower says, it could easily have turned out differently.