Miles Thomas Jensen wanted to stop his girlfriend's 14-year-old sister receiving obscene text messages from an older man.
But the Hastings man went about it the wrong way and ended up with a conviction for violence.
Police prosecutor Sergeant Fred van Duuren said a 20-year-old man had allegedly sent obscene texts and pictures, including images of genitalia, to the 14-year-old sister of Jensen's girl-friend.
Jensen heard about the incident through his girlfriend and decided to meet the man.
His girlfriend then texted the man, posing as a 15-year-old girl.
A meeting was arranged outside the Hastings Library at 7pm on Tuesday, March 14. There, Jensen's girlfriend pointed out the man.
"The defendant arrived and approached the victim and he grabbed him around the shirt front and abused him before punching him repeatedly in the face," Mr van Duuren said.
The text messager was banged against a steel rod and thrown to the ground, he said. Jensen forced the man to his feet and dragged him across the park to the police station.
"He deposited the victim in the foyer and told the attending officer what had happened."
The assault left the text messager with bruising and swelling to his face and he had to be hospitalised.
A well-dressed, tidy Jensen stood nervously in the dock while the summary was read out. He pleaded guilty to a charge of assault and Judge Tony Adeane convicted him.
Jensen will be sentenced on April 13.
The target of the attack is charged with sending obscene texts to Jensen's girlfriend and her sister, and will appear in Hastings District Court on April 6.
He has yet to enter a plea to three charges of doing an indecent act with the intention of offending.
Just weeks ago, text bullying hit national headlines when Alex Teka, 12, of Putaruru, was found dead in her home after being the target of text bullies.
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