A man sent 200 text messages to his partner's friends and family, the court heard, including threats to kill.
Rhys Carroll pleaded guilty in the Whanganui District Court on Tuesday for a charge of threatening grievous bodily harm and theft.
Police prosecutor Steven Butler told the court on October 8 the victim was leaving Carroll's home at the urging of her daughters.
As she attempted to leave, Carroll grabbed her phone off her and refused to return it.
Carroll then looked through the phone's contents and became enraged.