An aggrieved grandfather will take legal action against a woman who posted to Facebook to accuse him of inappropriately touching his grandson.

It could prove to be a "very costly post", according to a legal expert, who warns that too many people throw comments around online without realising the legal trouble they could get into.

Photo: File.

The woman who made the claim, on an open page, named the child and his grandfather and asked people to share the post.

The man mentioned in the post vehemently denies the claims.

"I would never, ever do that. My grandkids are my world. I'm broken, absolutely broken. I don't even know the woman."

The man's wife said that if the woman had serious concerns, she should have approached the police, the child's school, or the family.

"We will be pursuing the matter through the legal system," she said.

"This is defamation, it's absolutely not true. We have been in talks with our lawyer and will look to file a civil claim shortly."

The family has also gone to the police, who are looking into the matter. The woman who made the claim declined to comment when approached by Fairfax.

The ramifications for the woman who posted the content and those who shared the post will be a costly exercise, according to an expert in media law.

Ursula Cheer, a Canterbury University professor who specialises in media law, said the post, which has now been taken down, is a civil case waiting to happen.

"This is a very serious crime to accuse someone of, and without the defense of truth, it can be a very costly post."

Cheer said the woman could be in for an expensive battle, and further costs could be awarded because the grandchild was named. Any potential victim of sexual assault has automatic name suppression in the court system.

Cheer said lawsuits involving posts on social media are on the rise in New Zealand - many are taking civil action against those who have posted defamatory content about them online.

"The problem with social media is that some think it is just a chat with another person. It's not," Cheer said.

"Users of social media are publishers, and like the media, they are just as liable."

A defamation claim can be commenced even where the material is seen by just one person.

And those who share the post are just as liable.

"It's called the 'chain of publication'," Cheer said.

"Anyone involved in publishing the material is liable as well."

Removing the post would potentially lessen any damages if a defamation case were successful, but did not remove the threat entirely.

"It may reduce her liability, but that doesn't mean you can't be sued for damages."

A similar case in Australia, where a man was falsely accused of being a paedophile on social media, had damaging repercussions.

The man had taken a cellphone picture of himself with a Star Wars cutout of Darth Vader to send to his children.

A woman who saw him thought he was taking photos of her young children and snapped a picture of him, put it on Facebook and warned other parents about him, labelling him a creep.

The photograph went viral and the man was subjected to death threats.

Say what?

Defamation is the publication of an untrue statement that harms someone's reputation.

If people think they have been defamed, they can engage a lawyer to launch civil court proceedings, usually with the aim of getting a compensation payment and/or apology from the person who made the statement.

Once the preserve of newspapers and other media outlets, publishing also includes posting something on social media and also sharing it, even if you didn't write the original post.

The only absolute defence is truth and you have to be able to prove it in court, not rely on the fact that others are saying it, too.

Honest opinion can be a defence, but this can be difficult and costly to prove in court, especially if you don't have a track record of making comments on the issue. Joining an online bandwagon can get you in trouble.

Try to avoid defamation online

Don't call people liars, cheats, or accuse them of a crime unless they've been found guilty in court.

Don't share around posts from others that do this - that counts as publishing the statement and can add you to the list of defendants.