A Wellington IT professional was discharged without conviction after admitting 47 counts of possessing child sex images.

New Zealand has been called naive about the dark world of child sex exploitation, as a decision not to convict a man with a collection of child sex images is upheld.

The pictures, downloaded from a Russian website, included photos of children being sexually violated by adults, engaged in sex acts with other children, and being forced to perform sex acts on animals.

A 40-year-old IT professional, whose name is suppressed, was discharged without conviction in Wellington District Court in November, after admitting 47 counts of possessing objectionable material.

Police appealed against the decision, which has been upheld by Justice Rebecca Ellis in the High Court in Wellington.

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She accepted his lawyer's argument that he had an obsessive-compulsive disorder, and simply stored them away "like a stamp collection", never to be looked at again.

But Warren Ferdinandus, director of anti-child exploitation group Ecpat NZ, said New Zealanders were "naive" about the child sex trade in places such as Asia and Russia.

It came with "a lot of abuse ... a lot of rape, a lot of trafficking", he said.

Sensible Sentencing Trust founder Garth McVicar said he was appalled by the decision.

"I know this is only seen as looking at porn, but the image they look at reflects a real victim somewhere."

The decision showed why New Zealand had a terrible record of abuse against children and women.

"In situations like this you shake your head in horror, but when you get beyond [that] ... you've got to seriously question what on earth's going on.

"Is there a belief in New Zealand that this sort of behaviour is OK?"

Police, fronted by prosecutor Ian Auld, argued in their appeal that district court judge Peter Butler had carried out the balancing test during sentencing "incorrectly", and was wrong in assessing the offending as being at the "lower end of the scale".

But in a judgment released on Thursday, Ellis said she would also have ruled "narrowly" to grant the man a discharge without conviction.

The argument that he merely collected the images, as a symptom of obsessive compulsive disorder, was "unusual" but credible, she said. A pyschologist had assessed him as being addicted to pornography, but not attracted to children.

She went on to rule that the man had led an otherwise "exemplary life".

"He is a family man who is devoted to his two young children. He has formerly been the principal breadwinner."

She said a conviction would almost certainly be a "complete bar to his future employment in the IT field" and would have a severe impact on his family.

She found the case "difficult and finely balanced", but in the end believed a conviction would be "out of all proportion to the gravity of his offending".

However, she said she did not take the harm of child exploitation images lightly.

"I do not for one moment underestimate the terrible and lasting harm that is wrought by the child pornography industry, and those who support it – whether by viewing, downloading, distributing or creating such material."

The man's offending came to light after a USB with the images on it was found on a bus, and handed in to police.

Police found about 2412 images and one movie on it. They chose 47 images on which to base charges.

Defence lawyer Mike Antunovic​ argued that the man got no sexual gratification from the photos, and merely stored them away.

There was a huge range of mitigating factors, he said, including co-operation with police, a guilty plea, remorse, and strong support from his family.

Netsafe executive director Martin Cocker said the Department of Internal Affairs ran a web filter aiming to block child sex images from getting to New Zealand.

"When we know there are images on websites or there are websites that provide these images, they can be taken off in their entirety."

The filter couldn't stop it all, he said, but it made it more difficult for people to access the images.

"It's removing an argument that 'I accidentally joined one of these porn trading rings'. Because people have to take significant steps to become part of them."

In the past, people with an interest in child sex images were "relatively isolated" from each other. "What the internet has done has connected them together as groups and encouraged more trading of content."

Police senior media adviser Rachel Purdom said police were yet to decide whether they would appeal further.