False story alleging officer has sex with underage person is "particularly destructive", police say.

A false website story alleging an officer had sex with an underage Kaikohe local is "particularly destructive" for the image of police, a Northland top cop says.

The story, headlined 'Kaikohe local caught having sex with a police officer in the police station', was posted on a prank news website alleging a 15-year-old Kaikohe local was caught having sex with an officer. It includes a photo of a former Kaikohe officer standing outside the Kaikohe police station.

Inspector Al Symonds said the story was not true and was concerned about the impact these allegations may have on the officer pictured.

"Police are public figures and we're used to all the jokes about donuts and going into bakeries, but this kind of thing can be particularly destructive," he said.

The website the story is posted on a site which enables people to post fake news stories to prank their friends.

However, several people who have shared the story believe it is real. Of the 20 public shares the Advocate could see, 18 people thought it was true.

"When someone's face is banged across social media it can have a huge impact. We're trying to lift our game when it comes to sexual violence crimes and if this post stops even one victim coming in and talking to us, it is terrible," Mr Symonds said.

Mr Symonds said the officer pictured would be notified but he was not sure what kind of action could be taken.

"The problem is that the nature of the internet means it's easy for people to be anonymous," he said.

Anyone who sees the post should ignore it and not share it further, police said.