The security industry says security guards are increasingly using smartphones to photograph incidents while on patrol.
The security firm Cityguard is defending a staff member who photographed children it says had been skateboarding outside Newmarket Train Station.
A witness said the children were unsupervised and she did not see the guard ask for their consent.
Security Association chief executive Gary Morrison said photographs were helpful for providing supporting evidence for incidents.
"The security industry more and more frequently uses smart technology to take photos as evidential backup to the reports that they provide," Morrison said.
"So generally within the industry it is quite common practice - and probably increasing."
Security firms should be treating adults and minors differently, he said.
Smartphone technology had improved rapidly, and firms should get legal advice about taking and storing photographs to make sure they were acting within the law, Morrison said.
Wellington lawyer Amanda Hill, who specialises in privacy issues, said the incident in Auckland possibly breached the Privacy Act.
It is legal to take pictures of people in a public place.
But walking up to a person, putting a phone in their face and snapping a photograph was "quite a different scenario", Hill said.
The Privacy Act specified particular care must be taken with children and young people when collecting their information, she said.
The Office of the Privacy Commissioner said people with concerns should approach the security firm in the first instance but if they were dissatisfied to go to the commissioner.
Cityguard said the images were treated in the strictest of confidence and not shared beyond the guard involved and that guard's direct manager. It said images were deleted after a three months.
Separately the police, the Independent Police Conduct Authority and the Privacy Commissioner are all investigating the way police get, use and store pictures of young people and adults.
It was prompted by an RNZ investigation which revealed officers are approaching young Māori who had done nothing wrong and photographing them, collecting their personal details and sending it to a national database.