Moko has been living in the waters around Mahia for two years, often playing with swimmers, The Dominion Post reported Tuesday.
As he moves through adolescence, however, he's become stronger and more aggressive, New Zealand Conservation Department spokesman John Adams said.
Last month, an exhausted woman swimmer had to be rescued from a buoy after Moko refused to let her return to shore.
Dolphins live in complex social groups where hierarchies are created and sometimes tested by aggressive acts, Auckland University marine sciences professor Mark Orams said.
"When humans become part of their social group, dolphins will not differentiate between species," Orams said, adding Moko would be more inclined to find dolphins with which to live if people would leave him alone.