Daniel Watson was a storeman. He turned 30 in 2013 as he was arrested for the first time for crimes which Commonwealth prosecutors have called "unprecedented" in this country.

His online campaign of grooming, manipulation, threats, extortion, blackmail and demands for pornographic images – while pretending to be someone else – were "calculating and predatory", according to prosecutors.

Watson's youngest victim was a five-year-old girl in America. Most were aged between 12 and 15 and lived in Australia. He targeted 175 child victims, 71 of whom engaged with him.

Victoria Police who tracked him down and convicted him are now speaking out about the case to warn parents to stay alert and monitor their children's social media use.

In the end Watson was charged and jailed for extorting pornography from 43, soliciting from 10 and harassing 18 over a period of nearly three years.

The key here is that Watson used everyday social media sites and apps – Facebook, Kik, Instagram and Tagged were his favourites, but he also used video-messaging on Skype – to commit his crimes. He was not creeping around on the dark web. He was out in the open, albeit using fake identities.

His victims were targeted at home on their laptops and smartphones. "These girls are sitting in their homes surrounded by their family and they are under attack by this guy," says Detective Senior Constable Christine Stafford, who found and arrested Watson while with Victoria Police's specialist Astraea Taskforce, targeting online paedophiles.

"The one place you think your kids are safe. Watson's method was simple but devastatingly effective. He pretended to be a teenage girl and messaged other girls randomly, complimenting them on how they looked and asking if they wanted to send him any pictures.

"He, as a she, at first asked for relatively modest selfies, such as the young girl in her bra.That was the bait. He purposely did not target obviously attractive girls for this reason, figuring the flattery would work. Then came his switch. The demands escalated rapidly.

"He would threaten to expose that image to the girls' school or to their parents," says Stafford, demanding the girls send more explicit images.

In many cases, she says, he was able to easily find out where they lived or went to school through social media because the victims had not enabled privacy controls on their pages.

Then he would demand that contact between the two moved over to email. This was because he wanted a high volume of pictures and videos sent to him. It also meant that if he didn't already have personal details of his victims he could get them from the emailed images through metadata – such as GPS coordinates – embedded in them.

Social media sites in general strip metadata. The female names he used were Chantelle, Jasmine, Christine and Jazzy Sparks.

He amassed 1480 photographs and 41 videos. Some of the victims were in the US and UK; the rest were in Victoria, New South Wales, the Northern Territory and Queensland.

In person, Watson was an unappealing figure. As part of his defence he had lawyers claim he was bullied at school for having an unusually large head. The garage he lived in was squalid.

Towards the end of his three-year spree, Watson had cajoled a 14-year-old girl from the eastern suburbs of Melbourne to send him pornographic material. By now he was making specific demands of his victims – what to wear, what to do (which could involve masturbation scenarios with objects, or stripping) and where to do it (usually in their bedroom in front of a mirror).

He always insisted the girls looked happy and often requested what one judge described as "classic pornographic poses".

The 14-year-old had a little sister, he found out. She was 10. He threatened to go public with the images unless she joined in, and she did, for a while. But it scared her, and because she was just that little bit younger, she was able to tell her mother, who took them to police. The sisters handed over the email address they were sending the images to, and also the fake profile Watson was using.

Stafford tracked both, and Watson was arrested in his garage and his devices – a desktop PC and his two smartphones – were seized. But it wasn't just two victims. It looked like more than 100.

The problem now was to find victims and get them to make statements against Watson. Most didn't want to. Once they did begin to talk to more victims, it became clear that Watson was using material from existing victims to lure new ones into his web.

Lawyer Andrew Sprague from the Commonwealth Department of Public Prosecutions said Stafford started using signs. "Say he knows his next victim is a 'Karen' and he's already got images of a 'Melissa', he asks Melissa to holda sign telling Karen she is sexy. That would be very convincing for the girls."

He says his criminality was very calculated. "He would continue on with a person for six months. The most he went on with one person was 18 months. He would come in and out of their life saying 'I'm back, I want you doing this or that.' They are begging him to stop. But his dialogue back to them was horrendous. He was so relentless."

Two victims attempted suicide; one in Queensland and one in the United States. Sprague says the law over digital photographs, internet pornography and children on the internet was still evolving, but the internet and social media was a "new playground" for paedophiles.

"It is the place where children are hanging out and where they are accessible."

Watson was let out on bail after he was first arrested. As soon as he got home to his Broadmeadows garage he tried to contact one of his victims again to ask for more sexual images. He was arrested again and has been in custody ever since. His initial sentence of seven years was appealed by prosecutors and he is now in jail for ten years with a non-parole period of seven.

Court hearings heard Watson had a "deprived" childhood and his mother died young. He was a heavy drinker and suffered "sexual dysfunction," according to a judge, which manifested in paedophilia and paraphilia.

Detective Senior Constable Stafford said the case should be a warning to parents on two fronts. First: make sure your kids have adequate privacy controls on social media. And two – is checking their online life invading their privacy, or protecting them?