The headline on a poster showing four convicted Hamilton rapists could incite hate speech against Muslim people.

A poster with photos and details of four rapists with the heading "Beware Mohammed" has been plastered on a transformer at a busy Hamilton intersection.

The poster is not illegal, because the details are factually correct.

However, Canterbury University dean of law Ursula Cheer said there may be grounds to lay a complaint to the Human Rights Commission on the possibility of inciting hate speech.

The headline would also incite racial issues for innocent Muslims living in the area, International Muslim Association of New Zealand president Tahir Nawaz said.

"Whoever created the poster is judging all Muslims, all those who follow the Prophet Muhammad, for the actions of four men. That's not fair and not very good."

The poster, which is glued to a transformer near the corner of Alexandra and Collingwood streets in the CBD shows the pictures of convicted men Abdirahim Sheik Mohamed Guled (Mohamed Guled), Keyse Aiwi Abdi, Mohamed Essa and Mohammed Sahib.

All but Essa were working for taxi companies at the time of the offending. Essa posed as a taxi driver.

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It is not known who created the poster or how many have been posted around the city.

Cheer said that the case would be difficult to argue in any court.

"If it's factually correct and the convictions were quite recent, I think that there probably won't be a defamation issue and there probably won't be a privacy issue," Cheer said.

"Saying beware of Mohammed at the top could arguably be a form of hate speech. It could be argued that it is having a go at Muslims generally. There's legislation under the Human Rights Act that is aimed at hate speech. It's very hard to succeed with a complaint under that, though.

"You have to go to the Race Relations commissioner at the Human Rights Commission and make a complaint and the threshold is very high. You have to show a deliberate intention to degrade a group of people."

Cheer said that a court case last year could have been an issue if the four men mentioned represented historic convictions.

"There was a case last year in the High Court in which a judge suggested that criminals have a reasonable expectation of privacy in their convictions. Now that's never been the view that's been taken in the past.

"Everyone would assume that a conviction is a public document and that therefore means it's information that is already out there and you can publish it as long as you're accurate. I would agree with that, too.

"The law of privacy recognises even things like convictions can become private over time, even though they were public at the time. If, for example, you are convicted of something back when you're 20 and ... you never offend again and a newspaper digs it out when you're 40 and you're standing in the local mayoral elections.

"You could argue it was a minor offence and has become private information. However, because these four were convicted recently, it would be hard to argue."

Cheer said the poster could also be seen as racist.

"That's the other possibility, if it is seen as a possibly racist poster. If the four men are in jail, then what is the point of the poster?

"The main risks are: defamation first, if the facts are wrong; possibly privacy, but it doesn't seem likely in those circumstances; and possibly hate speech, but it's quite hard, as I say, to cross the threshold there. What they do initially is mediate to try to get whatever is stated taken down. If they can't, then you can complain further."

Nawaz said it would be difficult to lay a complaint to the Human Rights Commission because the author of the poster is unknown.

"Those that have done the crime have been judged by the justice system. Why are they targeting others? This will disrupt the community."

Police were not aware of the poster, but once told said they will not investigate it.

Human Rights Commission spokeswoman Michelle Taylor said the commission was unable to comment on the issue.