Hundreds of users of the social networking website Facebook could be sued for defamation for joining a group that claims David Bain is guilty of murder, a media law expert warns.
Mr Bain was acquitted last Friday by a jury in the High Court at Christchurch for killing his parents and three siblings.
However, a group called "David Bain is Guilty" has appeared on Facebook. It has more than 300 members.
Part of the introduction to the group site says "Join this group if you still believe in justice. . .."
One message warns users about not repeating suppressed evidence on the site.
When a Facebook user joins the group, a notification appears on their, and their friends' sites announcing it.
Wellington media law lecturer Steven Price said it was defamatory to say someone had committed a murder.
"The main defence would be, is it true? And if David Bain wanted to test that in court he could sue for defamation."
The person making the claim would then have to prove on the balance of probabilities, that Mr Bain did it, Mr Price said.
"And that's very different from a standard of reasonable doubt."
It would not matter whether people joined the groups before or after Mr Bain was acquitted of the crimes, they could still be taken to court, he said.
"I think more people are feeling freer to comment now, in part because we've got a verdict, in part because there is no danger of prejudicing the case.
"But, if they said it a week before the verdict, then he could sue for that now too."
The Facebook group also features a picture posted by one member of what appears to be a Tui billboard that says "Dad did it while I was on the paper round – Yeah Right".
Tui spokeswoman Jo Jalfon said the picture had been doctored and was not from the DB brand.
Mr Price said the doctored photo would also be potentially defamatory.
"It's possible that there may be another defence of honest opinion, which would be interesting. Truth is not the only defence of the Defamation Act.
"They could argue it was a joke or an honestly expressed opinion."
People could also be sued if they wrote on their personal Facebook page that Mr Bain was guilty.
"(Also) if they put it up on somebody else's site, then if David Bain wanted to sue he could sue either the person who posted it or the person who is hosting the site, or potentially the web server (ISP) itself," Mr Price said.
Also featured on Facebook is a group called "David Bain is Innocent", which has 123 members.
Another, "David Bain – Who Cares" has only two people supporting it.
In all, there are 21 groups associated with Mr Bain.
Of the more frivolous are: "Give David Bain His Paper Round Back", 47 members, "David Bain Sweater Appreciation Group", with 583 members, and "David Bain versus Mark Lundy", 36 members.
By a wide margin however, the most popular group on the website is "David Bain for Dancing with the Stars", with a whopping 3975 supporters.