A man who allegedly posed as Justin Bieber online to solicit explicit images from children has been charged with more than 900 sex offences, Australian police have said.

Gordon Douglas Chalmers, 42, a law lecturer at Queensland University of Technology in Brisbane, faces 931 child sex offences dating back a decade, including rape and making child exploitation material.

Officers in Queensland said he used Facebook and Skype to impersonate the star and warned fans and their parents to be extra vigilant when using the internet.

He was held after tip-offs from US and German authorities.

The man was already facing charges in Queensland of possessing exploitative material and grooming children, but the 931 charges were added this week after a “thorough examination” of his computer. That resulted in a large amount of child exploitation material being seized involving offending dating back to at least 2007, police said.

The charges include rape, indecent treatment of children and making child exploitation material.

The offences involve 157 alleged victims around the world, reportedly including up to 20 in the UK and 50 in the US.

Detective Inspector Jon Rouse said: “This investigation demonstrates both the vulnerability of children who are utilising social media and communication applications and the global reach and skill that child sex offenders have to groom and seduce victims.

“The fact that so many children could believe that they were communicating with this particular celebrity highlights the need for a serious rethink about the way that we as a society educate our children about online safety.

“The breadth of offences committed in this instance are frankly horrendous and I want to recognise the efforts and commitment of the investigative team at Taskforce Argos to keeping children safe.”