Organisations are typically oblivious to the presence of hidden cameras and, on the odd occasions they do find them, are reluctant to come forward to police for fear of reputational damage, said Julian Claxton, a surveillance expert whose company, Jayde Consulting, conducts sweeps for recording devices.

Just this year, Claxton has investigated two instances in Sydney, one involving a hidden camera in the change room of a Sydney private school and another involving a camera placed in the toilet of a building in Haymarket.

"Two or three inquiries a year where devices have been found is quite significant … in this business you would be lucky to find something 1 or 2 per cent of the time," said Claxton, estimating he saw only 10 per cent of the reported cases in Sydney.

"We believe that there's a lot more going on that's not being picked up."

He would not reveal the name of the school but said the school did not pursue the matter with police because it "didn't want the disclosure".

"One of the students found something sitting on the top of a rubbish bin and it turned out to be a little camera that was recording to an SD card," said Claxton.

In the Haymarket case, a woman found pictures of herself sitting on the toilet. The images had been floating around her local community.

Although publicity of these cases is rare, two have been reported this month.

The Australian Defence Force Academy charged a 21-year-old male cadet with secretly filming a female colleague in the shower. The man had hidden a mobile phone in a vent.

On August 24 police released pictures of a man they believed planted a mobile phone in a Sydney clothing store change room. Police warned people to be on the lookout for any recording devices when they use change rooms in retail shops.

In 2008, an SBS staff member was suspended after photographs of three women taken in an SBS change room were found on his personal computer. The camera had been hidden in a ceiling and the perpetrator was caught only after the photos were discovered by a PC repair shop.

Claxton said that, while mobile phones were easy to spot, there were scores of tiny recording devices available on eBay that were small enough to fit into the head of a screw, buttons, pens or cigarette packets.

"One-inch squared would be a big one," he said.

They have become so advanced that one 2.5" camera available on eBay has a wireless transmitter on it enabling the offenders to view the footage from a distance without ever having to retrieve the device. The cameras can either be connected to small batteries or permanent power sources.

Claxton said the problem was not only compounded by the fact that cameras were getting smaller, but also by the ability to record significantly more footage on a single card than ever before.

"They are coming in so many different sizes, shapes and forms now that, for $10, I can buy a camera hidden inside a writing pen with a USB solid state recorder inside," he said.

"It'll give me several hours of recording in my pocket or I can leave it on a desk or I can prop it up on a window sill in a shower."

He said the only way to catch the perpetrators was to sit and wait for them to come and retrieve the camera or catch them in the act of installing it in the first place.

"In most cases when we do find listening devices it's very, very difficult to actually catch the perpetrators despite its illegality," said Claxton.

"I would suggest that there's a lot more that goes on where they throw something in for a period of a day and then they pull it out that night to get whatever their thrill is and then they will try that again down the track."

He said he recommended that clients modify the environment to make it more difficult for cameras to be installed, such as only using solid ceilings, choosing vent locations well and making sure things such as vents and soap dispensers are not easily tampered with.

eBay's head of communications for Asia-Pacific, Daniel Feiler, said that, if the government introduced legislation banning the sale of hidden camera devices, eBay would look at getting them removed but as of today they were legitimate products.

"I don't see why we should be held to a higher standard than an offline or any other online person who can sell them. For the most part they're used legitimately," he said.

NSW Police has been approached for comment. Comment is also being sought from David Jones and Myer regarding how often they sweep for hidden cameras.

This reporter is on Twitter: @ashermoses