A man has been jailed for watching cartoon videos of elves, pixies and other fantasy creatures having sex.

Ronald Clark downloaded the Japanese anime cartoons three years ago, setting in train events that would see him in court in Auckland and jailed for three months for possessing objectionable material, and sparking debate as to what harm is caused by digitally created pornography.

Clark has previous convictions for indecently assaulting a teenage boy and has been through rehabilitation programmes, but the video nasties he was watching in this case were all cartoons and drawings.

He says the videos came from an established tradition of Japanese manga and hentai (cartoon pornography), a massive, mainstream industry in that country.

They weren't even depictions of people - Clark's lawyer Roger Bowden described them as "pixies and trolls" that "you knew at a glance weren't human".

Bowden said the conviction for possessing objectionable material was "the law gone mad".

However, while the cartoon characters were elves and pixies, they were also clearly young elves and pixies, which led to concerns the images were linked to child sexual abuse.

Anti-child pornography group ECPAT Child Alert director Alan Bell said the images were illegal because they encouraged people "to migrate from there to the real thing".

"The distribution of it is damaging. You have to ask what impact does it have even if it's not harming [an individual child]."

Bell said it had to be conceded that no child was harmed in the images' production but "it's all part of that spectrum". Cartoon images of child abuse were a "huge" problem in Japan and the practice had started finding its way into computer games, he said

Lincoln University philosophy lecturer Grant Tavinor, who writes on the aesthetics of video games, said the case raised two key questions: Did producing the pictures harm anyone, and could their viewing and distribution be injurious to the public good?

"The worry is that viewing or distributing such images could support the sexual exploitation of children even if the production of the images did not actually involve the exploitation of any children," Tavinor said. It's not enough that no one was harmed in the making of the videos, the law takes a protective role and says there are some things we just don't want circulating in society, he said.

Auckland University associate philosophy professor Tim Dare said "the justifications for punishment are likely to be worries about the tendency of the images to promote harm to real people in the future, or a concern for what the interest in the images tells you about their ‘character' ".

Clark himself argued that the law led to the absurdity that he could, in theory, be convicted of possessing objectionable images of stick figures.

Clark admitted he was interested in the images but he said it was for their artistic merit and as "a bit of a laugh". He did not find them sexually arousing, he said.

Tavinor said there were ethical issues that complicated the case.

"The ways a person entertains themself is not morally negligible. This is probably an additional factor in the current case because as well as worrying about the effects these activities might have on children, we also naturally make moral judgments about the character of the person in question.

"But for the purposes of law it is probably important to distinguish between these because convicting someone for their moral views is very dangerous."