A 34-year-old Whangarei man sent more than 2000 explicit and threatening text messages to a 13-year-old girl.

Aaron Tucker eventually slept with the girl and was this week sentenced at Whangarei District Court to four years' jail on three charges of unlawful sexual connection with an underaged girl.

The case has prompted NetSafe, an organisation that promotes safe use of cyberspace, to urge parents to access information from their children's cellphones and computers to ensure their protection.

Judge John McDonald said he failed to understand why Tucker was not charged with rape because the girl was clearly pressured into sex.

"Why does a 34-year-old man in a relationship need to prey on a 13-year-old girl?" he asked.

In January 2010, a girl known to the victim began receiving texts from Tucker. Tucker began targeting the 13-year-old victim after she told Tucker to leave the other girl alone.

He sent more than 2000 texts, most of a sexual nature, to the victim over two months.

In one instance, Tucker asked her to send nude pictures of her and he would send his.

"The text messages went to her day and night, including when she went to school," the judge said.

"In the end, she gave in due to the frequency of the texts and pressure on her."

On March 2, Tucker picked her up and drove her to an isolated area south of Whangarei where he had sex with her.

He repeated the acts on two more occasions before he was arrested.

Tucker initially told police he thought the girl was 17, then 15 and finally admitted he knew she was 13.

Defence lawyer Chris Muston said Tucker accepted responsibility and understood the hurt his actions caused his and the victim's families.

Netsafe executive director Martin Cocker said cellphones were being significantly misused.

"We'd encourage parents to have an idea of what their children use cellphones for because they have a right to access information from their cellphones for their protection," Cocker said.