Undercover police officers in Australia ran the world's largest child sex abuse site for nearly one year in a bid to infiltrate to the top of the administration and arrest perpetrators, it has emerged.

Queensland Police unit Taskforce Argos took over 'Child's Play' website last October, after its founders were arrested in the US.

Police officers Jon Rouse and Paul Griffiths impersonated the website's founders for 11 months, engaging with paedophiles and sharing material online in a bid to catch perpetrators. Their work was part of 'Operation Artemis', a joint investigation involving Australian, American and European authorities.

The undercover operation resulted in the identification of 90% of the users and 1,000 arrests.

The website, now closed, had more than 1 million profiles and was available on the dark web, a section of the internet that people can theoretically use anonymously.

The revelation was made by Norwegian website VG, which was conducting its own investigation into Childs Play's origins and its traffic.

The newspaper had found out about the undercover operation in January, but decided not to publish the finding of its own investigation so that authorities could continue their work.

The task force was criticised for sharing videos and photos of children being abused during their operation.

The mother of one of the children whose pictures were shared on the website said she did not agree with the methods of the investigation.

"My daughter should not be used as a bait," she said. "If the pictures of her are shared on the website... she should be paid or get compensation for the use. It's not right that the police should promote these pictures."

However, officers explained that sharing material was necessary to achieve the investigation's goals and ultimately close down the site.

"We have one goal and that is to stop the sexual abuse of children. We will do whatever we can within our legislative authority to achieve that," Rouse told VG.

"Sharing any such image is an abuse of that child. However, it is something we can justify as being for the greater good and to prevent ongoing abuse of children. I hope they understand that we are trying to catch as many offenders as possible."

Griffiths explained that, under Australian law, a judge could grant police officers special permission to commit criminal acts that would be prosecuted in normal cirrcumstances.

"Under a so-called controlled operation, we get permission from a judge to be allowed to act in ways that would normally have been considered violations," Griffiths said. "We are entitled to commit certain criminal acts and we are exempted from prosecution because we investigate these specific crimes."

He also said that the pair had to replicate the previous administrators' language, including the use of emoticons, in order to convince users that they were still engaging with them.