He said criminals posed as children on social network websites to find victims who take explicit pictures of themselves.
"It's often associated with teenage risk-taking behaviour.
"But the problem is that once you take an image of yourself and send it to someone else, you lose all control over it and what that does is it opens the door, potentially, for people to say, 'You sent me this image and if you don't send more explicit images, then I'm going to post all of these images to all the people that are in your social networking groups'."
Mr Michael said in many cases, the abuser was known to the child.
A Department of Internal Affairs spokesman said images were getting more violent, showing people being raped or tortured.
The department uses undercover agents to go online to flush out offenders.
