LEIGH SALES, PRESENTER: It's not often that you get intimate access inside a police sting that brings down a global crime network. But that's what 7.30's bringing you tonight.

Last year, a finely-hatched police operation netted not only the Australian head of a global online paedophile ring, but also senior members of the international network.

Officers covertly infiltrated the group. It was involved in the abuse of hundreds, possibly thousands of children around the world.

Our reporter Mark Willacy was shown inside this unique investigation. Here's his report.

MARK WILLACY, REPORTER: In the virtual world, he was the undisputed leader of the globe's biggest child abuse site, an online network of paedophiles in dozens of countries.

PAUL GRIFFITHS, TASKFORCE ARGOS, QLD POLICE: Everything from babies and toddlers right up to what they would describe as jail bait.

JON ROUSE, TASKFORCE ARGOS, QLD POLICE: The material that was particularly being produced by this group, we knew was happening almost live and on demand.

STEPHEN HEGARTY, SOUTH AUSTRALIA POLICE: He is a predator. Um, yeah, an extremely dangerous individual as far as I'm concerned.

MARK WILLACY: To catch this predator would take a unique operation involving the co-ordinated efforts of police from Germany, the Netherlands, Denmark, the United States and Australia who would piece together the man's identity one clue at a time.

PAUL GRIFFITHS: We knew information about the administrator of the site and we knew - there were certainly significant suggestions that he was an Australian.

JON ROUSE: As soon as we had a sniff that we were dealing with an Australian who was potentially the lead administrator, well that presents a challenge that we were prepared to take on.

MARK WILLACY: The head administrator presided over a site that was strictly hierarchical, with members who shared original material of them abusing children promoted, while others were punished for not meeting expectations.

STEPHEN HEGARTY: So to be a member, you had to have access to child exploitation material and be posting it onto the site for others to have, and that had to be an ongoing thing, and if you'd failed to do so, you'd be removed from the site.

PAUL GRIFFITHS: The level of material, the amount of material you posted was significant enough, high enough amount of material, then you could apply to move up the ranks and become a VIP.

MARK WILLACY: There was no bigger VIP than the head administrator and the suspected Australian had one of the world's best victim identification specialists on his trail.

PAUL GRIFFITHS: I had photographs of him potentially abusing seven different children, all pre-school age, boys and girls, different ethnic backgrounds. Somewhere at the back of your mind there's something saying, "Who the hell is this guy that he's got access to those children?"

MARK WILLACY: For 20 years, Paul Griffiths has trawled through images of child abuse for clues to the identities of victims and perpetrators. To unmask the head administrator, Griffiths would have to piece together a scattering of leads, among them, an unusual greeting the man used online.

PAUL GRIFFITHS: And one of the things that I noticed was that on a number of occasions, he's used that specific greeting, "hiya's" - H-I-Y-A-S.

MARK WILLACY: After ruling out women who used "hiya's" as a greeting and after sifting through thousands of sites and discussion boards, Griffiths found a man who consistently used the word. That led to a Facebook page, a photo of a VW utility and a city.

PAUL GRIFFITHS: I mean, the Facebook profile was completely bogus, but it did have that clue for Adelaide. It did point us in the direction of Adelaide. I actually phoned him asking questions online about how to raise the suspension on his four-wheel drive.

JON ROUSE: He made mistakes, but any one of those mistakes in isolation was not enough to identify him.

MARK WILLACY: But the VW ute would help piece together the jigsaw.

ACTOR (re-enactment): He's actually posted a photograph of his vehicle and he also signs off his messages with the name Shannon. So from the information we've got from that registration plate, it would appear to be registered to someone by the name of Shannon McCoole.

MARK WILLACY: Queensland's Taskforce Argos finally had a name. It was time to brief their counterparts in South Australia's Sex Crimes Investigation Branch.

The risk was massive because 32-year-old Shannon McCoole was charged by the state with looking after dozens of vulnerable children and he'd been proudly posting evidence of his own sexual abuse of those in his care.

ACTOR II (re-enactment): Looks like he's also employed with Families SA as some sort of care worker in the southern suburbs.

ACTOR III (re-enactment): That's they obviously quite concerning, I imagine definitely at your end.

ACTOR (re-enactment): I can see so far there seems to be seven different children, so three girls and four boys and we're talking pretty hardcore material on all of the children.

STEPHEN HEGARTY: It was probably the worst I've ever seen. You know, the material's horrific. There's no other word for it. He's probably had access to hundreds of children.

MARK WILLACY: With McCoole able to access children at will, the Queenslanders and South Australians co-ordinated a plan.

ACTOR IV (re-enactment): Obviously we have to move as quickly as possible re: children at risk.

ACTOR II (re-enactment): Getting a live capture is critical.

ACTOR V (re-enactment): Well basically, John, as soon as you guys can get down here, we'll be going in. Obviously what you've said, it's imperative that, you know, we get him whilst he's got his computer running.

MARK WILLACY: Four days later, police moved in.

STEPHEN HEGARTY: Myself and a colleague attended his address at 7 o'clock at night and it was as simple as knocking on his door. He opened the door and before he had the opportunity to do anything, he was secured in the premises. He was a bit shook up and very quiet. There was a point he was almost physically sick at one point.

MARK WILLACY: It was from this house in suburban Adelaide that Shannon McCoole secretly ran a global internet child pornography ring that had tens of thousands of members. Rising to head administrator, McCoole had complete control over the network, a role only made possible by his unfettered access to children in his care.

The public servant, who'd repeatedly abused vulnerable kids as young as 18 months old, was quickly identified by his own depraved internet posts.

STEPHEN HEGARTY: We had some images of been individual we believed to be him, the individual from the site, sexually abusing children. He had distinguishing mark of a freckle on his finger.

PAUL GRIFFITHS: I think it became apparent when we asked to look at his hands and particularly when his hands were photographed and the famous freckle on his finger was pointed out and confirmed.

MARK WILLACY: Shannon McCoole was in custody, but Taskforce Argos wasn't finished. It was time for phase two of its operation.

JON ROUSE: Phase two, for me, was to take over the network, assume control of the network and ultimately identify as many criminal offences committed by members of the network as we could before ultimately destroying it.

MARK WILLACY: But infiltrating and running a global child abuse site presented a dilemma for Argos: how to keep the network going without allowing the sexual abuse of children to continue?

JON ROUSE: There was just no way on Earth that we are going to allow the sexual abuse of children to continue. So we closed membership. Nobody gets in. Effectively, we caged the rats.

MARK WILLACY: Two Taskforce Argos detectives pretended to be Shannon McCoole, 24 hours a day, seven days a week.

JON ROUSE: And it can mess with your head. So, it's not something you want to do for really long, protracted periods of time.

MARK WILLACY: But this covert operation would go on for 10 months and the Argos infiltrators would help authorities round up paedophiles across the world, from the United States to Europe and Asia.

JON ROUSE: We're stretching the bounds of what has ever been done before.

MARK WILLACY: As each of these caged rats was rounded up, they were replaced by undercover police. But Jon Rouse refuses to measure the operation's success in arrests.

JON ROUSE: I think in terms of the identification of child victims, that was - that's what it was all about. There was a lot of kids that are in a better place now because of what happened across the globe.

MARK WILLACY: Potentially hundreds of kids?

JON ROUSE: Yeah.

MARK WILLACY: This month, Shannon McCoole was sentenced to a record 35 years in prison, with a non-parole period of 28 years.

STEPHEN HEGARTY: Sometimes things are a little bit beyond comprehension, to be honest. But that is how calculating he was. So, um - yeah, I - sometimes it's very hard to understand how he could've done what he did.

PAUL GRIFFITHS: We see ourselves as the lucky ones in that we can actually do something about it rather than just sitting back and thinking, "Oh, that's terrible and there's nothing we can do."

LEIGH SALES: Mark Willacy reporting.