A video shared to Instagram in February, and reported to the Department of Conservation, showed a man jump from a boat into the sea off the coast of Devonport near two orca.
DOC principal investigation officer Hayden Loper said it displayed a shocking and stupid attitude to protected marine mammals.
"The video left us genuinely stunned," he said.
"As well as the initial attempt to dive onto the animal, the man stays in the water and then swims toward it again in a second attempt to touch it."
Loper said it was "extremely irresponsible".
"This is stupid behaviour and demonstrates a shocking disregard for the welfare of the orca.
"Orca are immensely powerful animals, and this really could have ended horribly - with either the startled whale being injured, or the man responsible being harmed by the aggravated animal.
"It's a very clear breach of the Marine Mammals Protection Act. Orca are classified as whales under conservation legislation and it is illegal to swim with whales, or disturb or harass any marine mammal."
Loper urged social media users to continue alerting DOC to content they believe breaches legislation protecting New Zealand wildlife.
"This is the third case in recent years in which social media content has led to a successful prosecution for DOC and we greatly appreciate the tip-offs we get from the public," he said.
Anyone who sees an incident they believe may breach conservation legislation can call 0800 DOC HOT (0800 362 468).
