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.

The old law dated to 1908 when it was believed women were not capable of sexual offending.

But The Crimes Amendment Bill No 2, introduced to Parliament in 2003, made sex offences, excluding rape, "gender neutral". It was passed in May last year.

This meant women could be charged for having sexual intercourse with boys, just as men have been charged for having sex with a minor.

A 23-year-old New Plymouth woman was among the first to be prosecuted under the new law.

Briar Jayne Dravitski pleaded guilty last month to two charges of an indecent act on a boy under 16.

The woman had sex with a 13-year-old who had been staying at her house after he ran away from home.

Dravitski said she thought the boy was 17 and that he had been thrown out of home.

She is due to be sentenced on January 17 and faces a maximum penalty of seven years' jail.

The most famous case of a woman jailed for sex with a minor is America's Mary Kay Letourneau. The 43-year-old was freed from jail in 2004 after serving seven years for having sex with Vili Fualaau. The sex started when he was just 12. He's now 21 and they have two children.