Young men caught having sex with an underage girl are likely to end up with a serious convictions that could have lifelong consequences, a Nelson detective is warning.

Detective Sergeant Ian Langridge said many young people did not realise a conviction of that type could prevent them from working in schools, as a care provider, or stop them from travelling to some places overseas.

The conviction was also a first-strike offence.

This meant those convicted would get a warning and a second serious offence would usually lead to a jail term with no parole.

For a third serious offence, offenders would get the maximum prison term for that offence, with no parole.

Even if the girl was nearly 16 and her boyfriend was a few months older, he could be charged, Mr Langridge said.

There have been a number of cases before the Nelson District Court in the past couple of weeks where men aged 19 or 20 have been charged with having sex with underage girls.

The sentencing judges have made it clear that the law is there to protect young girls from older men, but also from themselves.

Girls under 16 are not considered emotionally mature enough to cope with being sexually active.

Mr Langridge said people were getting carried away without thinking the consequences of their actions through.

"The consequences can be quite harsh for all involved. All they have to do is take a step back and consider the consequence of their actions."

A new child protection unit was recently set up where detectives work alongside Child Youth and Family staff to help tackle child abuse and protect children in the region.

Mr Langridge said it was his feeling that police were dealing with more people charged with having sex with someone under 16, but it could be that the new child protection unit had more time to deal with those cases.