The law in Australia is struggling to keep up with teenagers having underage sex and using mobiles to send explicit photos to each other.

Lawyers and judges in South Australia said they were seeing an increase in sexting and underage sex cases coming before the courts.

Several judges in South Australia spoke out recently about the need to adjust the law so that young people in consensual relationships did not end up on the child sex offenders list.

The Law Society of SA launched a smart phone app late last year called The Naked Truth to help educate young people about laws regarding sexual activity.

Justin Simmons was part of a group of young people who helped with the development of the app and said teenagers sending naked photos of themselves to one another - or sexting - is common.

"It's one of the most common things that teenagers do with their phones, I think," he said.

"There are people who will happily send stuff to each other with no idea of the repercussions."

AJ Reigh also helped with the app and said rules including the different ages of consent could be confusing.

"Knowing the laws really would have saved some of my friends from making bad decisions, without going too far into detail, I think you know what I mean," he said.

So far the app has been downloaded 3,500 times.

Former president of the Law Society of SA Morry Bailes said the feedback so far has been positive.

"[We've had] comments such as the fact we've done something that the government itself should have considered doing," he said.

"So we do feel it's innovative and it is pitched in a way that we hope younger people will use and find attractive."

Judges call for changes to SA law

Several district court judges in South Australia have spoken out recently calling for changes to the law.

At the moment, any person over the age of 18 convicted of a sexual offence ends up on the sex offenders registry.

Judges and lawyers said there was an increase in the number of cases involving consensual relationships where one person was just underage and another just over.

Mr Bailes said sometimes the older party ended up in court on child pornography or unlawful sexual intercourse charges.

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"We really question whether or not the criminal court is the right jurisdiction for those matters to end up," he said.

"These laws are intended to stop predatory behaviour against children and it's essential that we have them, however, we don't really want to see children before the criminal court, potentially getting onto the sex offender register in circumstances where really they needn't be there."

The senior judge of South Australia's Youth Court, Stephen McEwen, said more cases of child pornography due to sexting were coming up.

"It's certainly a social issue that needs to be addressed that probably does overlap with the criminal law, but whether it's squarely within the criminal law I think is probably a moot point," he said.

Judge McEwan said it was a very difficult situation for legal professionals.

"The law is able to adapt, it takes time and some would say the law takes too long, but when you're bringing in criminal sanctions you need to hasten slowly," he said.

"In South Australia there have been some adaptations and amendments already, [but] as in most issues, the criminal law is not going to have all of the answers."

He has commended The Naked Truth app and said more education was needed.

"On the one hand young people are incredibly perceptive and insightful and intelligent in using the technology, but on the other hand their decision making about using the technology to put certain images out there strikes me as just incredibly unperceptive, unintelligent and lacking in good judgement."