They knew she was Australian with some certainty – Australian school uniforms are, apparently, unique.

But they did not know exactly where she was, when she'd been abused, or who had abused her. It was early November 2014.

About the same time, in a regional Victorian town, the abuser was huddled around a computer of his own.

On hard drives attached to the computer were about 17,000 child abuse images and videos he had produced, and shared on the dark web using the Tor network. There were not only images and videos of the primary school student, but her sister.

And there were pictures of his surrogate twin daughters, whom he had abused for seven months, starting from when they were 27 days old.

They were not to know that, of course, at The Hague. But they knew the clock was ticking.

The image of the schoolgirl was sent to the Australian Federal Police, and Operation Aqueous was launched.

Detectives found the NSW school that matched the uniform, and then the girl who matched the photo, and then the house that matched the girl.

It was raided within three weeks of the photo being uncovered, and police, after investigating who she lived with, suspected her father was the abuser.

Soon after the raid, it became clear that he was not their man. And he also did not know who the abuser could be.

The father, and his wife, had two girls, aged nine and 10.

It was not until they saw an image of their daughter, taken from the same collection as the school uniform photo, at a wildlife park, that they twigged. They knew that on that trip, the girls had been with their uncle.

The girl shown in the image at The Hague had been identified, but the number of victims had seemingly doubled – to the girl and her sister. That number was about to double again.

Police found that the girls' uncle fathered twin surrogate daughters in March that year.

It was a stomach-turning discovery. The clock ticked even louder.

On December 9, about a fortnight after the NSW raid, the AFP stormed the property where the man lived with his wife, and his twin girls. They were almost nine months old.

The man had no criminal record. He was a well-loved husband who had a stable upbringing, and was a popular colleague at the place he worked for more than 20 years.

But he had an odd side, too. He had been an underwear thief  as a teenager. Later, he started cross-dressing, insisting on wearing women's clothing while making love to his wife.

Their sex life had ended more than a decade before his arrest, and they had slept in separate bedrooms. In his bedroom, he kept a computer.

In the County Court last Thursday, Judge Susan Cohen sentenced the 49-year-old man to 22 years in prison, with a minimum of 15½ years.

He pleaded guilty to 38 charges including two of trafficking children, 20 of incest, 11 of producing, accessing, or transmitting child abuse material, and an upskirting charge relating to photos he took of women's underwear while riding V/Line trains.

Judge Cohen viewed some of the images the man stored on his computer before sentencing him, after a submission was made by the prosecution.

"The fear I saw in the eyes of an unknown very young child ... made a deep impression on me that will be very hard to forget," she said.

She was particularly troubled by the man's spiral into some of the most depraved offending ever prosecuted by the AFP.

He had gone from a casual viewer of child abuse material, to an active participant in online chat forums, where he pretended to be a woman with the user name Candy.

Then, when he reconnected with his estranged brother in the mid-2000s, he was trusted to care for his nieces, and regularly took them on outings and bought them gifts.

He started abusing them in 2009, and his offending escalated until he started drugging them before they were abused.

Judge Cohen said organising the birth of other children to abuse showed the hold his "warped desire" had on him.

A desire that started, he says, when he accidentally clicked on a link in about 1994.

"These sorts of cases show what child exploitation material online can cause," AFP Commander Glen McEwan said. "This offender has gone from 0 to 300 miles an hour in a moment."

Commander McEwan, the manager of the Victim-Based Crime Command, said that despite the work done at The Hague by the Europol-hosted Victim Identification Taskforce, it was clear policing would not solve a growing appetite for child abuse material.

"It is such a wicked problem," he said.

"Even though we have prosecuted this offender, there is no good news story. Not for the victims, or the wife, or the parents of his nieces, or the police, or the court.

"The reality is that this is just part of the solution. This is an ingrained behaviour that we as a society have to work together to try to understand."

Part of the answer, he said, could be understanding what draws child abusers into the online community that often fuels an escalation of their offending.

In organised crime, the motivation is money. In child abuse, the motivation is belonging to this community.

"There is a one-upmanship about the behaviour on these forums. And there's no end game. It is about producing and viewing material for their own gratification and the gratification of others."

Operation Aqueous detectives uncovered 59 other victims of child abuse from the material seized from the man, and 30 victims – including the four Australian girls – were identified at The Hague.

And while they are now safe from harm, the images of them online may circulate forever.

As the man was sentenced, he stood in his dark suit, his left hand clasping his right wrist.

The eyes of former workmates and friends, who had routinely turned to glare at him during the hearing, and muttered their disgust, were now completely fixed on him.

They were pitiless, unmoved even after hearing that he had attempted suicide.

Judge Cohen said his sentence should not be too crushing, as he should have some motivation to reform himself, and a chance to rebuild the relationships he had shattered. But she conceded this might be unlikely.

"You face a long and lonely time in prison."