Police are investigating claims that a 23-year-old Auckland woman had sex with a 13-year-old boy she was supposed to be babysitting.

The mother of the boy says she was devastated to find out about the relationship, after intercepting her son's text messages.

The babysitter, who used to share a house with the family, was left to care for the boy and his younger brother while their mother worked at night. The mother became suspicious of the relationship and swapped numbers on her son's cellphone. While the boy thought he was texting the babysitter he was actually texting his mother.

"When I found out I was absolutely devastated to know this woman had my son, my baby, in her bed. It's disgusting," the woman, who cannot be named for legal reasons, told the Herald on Sunday.

"The woman got pregnant and my son was texting her, worried about whether he was the father or not - this is a 13-year-old boy worried about something like that.

"He fell for her but he is 13 - what 13-year-old wouldn't fall for someone older if they paid them that much attention?

"It was in her power to make sure this didn't happen. It's illegal."

When confronted by his parents, the boy admitted having sexual intercourse with the woman while his mother was at work and his young brother was in another room.

When the mother became suspicious of the relationship, she warned the babysitter to distance herself from the boy.

"He would go into her room when he was supposed to be brushing his teeth and be on her bed. They were spending a lot more time together," the woman said.

"He even bought her a pretend diamond ring which she wore - she should never have done that."

The mother told the babysitter to stay away but she was assured there was nothing going on. "She told me I was lucky my son had someone to talk to and I felt guilty for suspecting something - then this happened."

The woman could not be reached for comment.

The interviewing officer in the case, Detective Mark McDonald, said it was a crime to have sexual intercourse with anyone under 16.

"The laws have changed so it doesn't matter whether the offender is male or female. The crime is just as serious as if it was an older man having sex with an underage girl."

Manukau City child abuse team head Detective John Timms confirmed they were investigating.

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Consensual sex with boy under 16 now a crime

Three years ago it was not an offence for a woman to have consensual sex with a boy under the age of 16.