In February, a man allegedly held his mobile phone under the cubicle wall of a Melbourne sports store change room to film a 32-year old woman trying on clothes.

Last month, Tegan Portener filmed a man who was allegedly filming her with his mobile phone from under the seat on the Sydney-Newcastle train. She gave the evidence to police, who arrested him. Her video went viral.

This week, Pedro Suarez was arrested after allegedly prowling Wollongong Central shopping mall attempting to film up several women's skirts with his mobile phone.

With an estimated 800 million individual porn pages on the internet, most of them free, there are easier, legal ways to get indecent pictures. But it's also easier than ever before to photograph victims covertly, because, thanks to smartphones and tablet devices, it's normal to walk around unnoticed with a camera in your hand.

"In clinical practice, we have seen trends increasing in these behaviours," says Georgina O'Donnell, a forensic and clinical psychologist with experience in cases of paraphilic disorder, the clinical category that covers so-called upskirting offences.

It's probably not that more people have voyeuristic or fetishistic desires than before, "rather the accessibility of the technology has just made it easier for offenders to take pictures when they otherwise may have only tried to peek", Dr O'Donnell says.

In NSW, the number of upskirting charges tripled in the three years to 2014. In the year to September 2011, there were 32 charges under Section 91L of the Crimes Act - "film a person's private parts without consent" - but 100 for the same period in 2014.

The situation was similar in Victoria, where 53 "visually capture person's genital/anal region" offences in 2011 ballooned to 139 in 2014.

Laws specifying upskirting as an offence have been introduced in Victoria (2007), South Australia (2008) and the ACT (2015).

Consent is everything. It is the difference between Marilyn Monroe exclaiming, "Isn't it delicious" when a subway breeze and a camera catch her skirt up on Lexington Avenue while filming The Seven Year Itch, and the distress or even psychological trauma felt by a woman who discovers a stranger has photographed her private parts without her knowledge.

Offenders experience "recurrent and intense sexual arousal and fantasy focused on the excitement of taking photos of unsuspecting females", Dr O'Donnell says.

They may keep them for their own masturbatory fantasies or share them with other collectors.

To be diagnosed with a paraphilic disorder, the person must feel personal distress about their atypical sexual interest or behaviour, or it must involve a threat to the wellbeing of others, or involve people unwilling or unable to give legal consent, says the latest version of the influential Diagnostic and Statistical Manual of Mental Disorders (DSM-5), published by the American Psychiatric Association. While there may be dozens of paraphilic disorders, the DSM-5 specifies only eight: voyeuristic, exhibitionistic, frotteuristic, sexual masochism, sexual sadism, pedophilic, fetishistic and transvestic.

In Victoria, recorded offences for taking upskirting images fell by half in the year from 2014 to 67 in 2015.

A similar drop was recorded in NSW, from 100 in 2014 to 67 in 2015.

But "we can't really know whether a reduction in the 'visually capture' offence is a true reduction, or just a reflection of better cameras and recording devices which are less detectable", says Anastasia Powell, a criminologist and senior research fellow at RMIT University who studies the role of technology in sexual violence and harassment against women.

In Victoria, there was a big jump in the separate offence of distributing such images, from three in 2014 to 17 a year later.

"You don't have to look far online to see that upskirting images are in high demand," Dr Powell says, with many websites and online forums trading in them.

The drop in the offence of taking images and the jump in the offence of distributing them "might suggest that the distribution and threats to distribute intimate images is emerging as a larger social problem than traditional upskirting", Dr Powell says.

Dr O'Donnell fears the widespread practice of "sexting" and the use of apps such as Snapchat to exchange explicit photos are desensitising teenagers to the personal boundaries and privacy that previous generations expected around photography.

Dr Powell says the crime-report statistics are likely to capture only a fraction of the problem.

"We have to remember that the very nature of upskirting offences is that they are most often taken without the victim's knowledge."

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