Don’t just take my word for it, let’s see what the Department of Internal Affairs has to say about how well their system works (the following quoted text is all from the draft Code of Practice):

ISPs Might Not Participate

Participation in the Digital Child Exploitation Filtering System by ISPs is therefore voluntary and this provides an effective means of ensuring that the system keeps to its stated purpose. If ISPs become uncomfortable with the direction of the system, they can withdraw.

Doesn’t Prevent the Creation of Illegal Material and the Exploitation of Children

The Department of Internal Affairs appreciates that website filtering is only partially effective in combating the trade in child sexual abuse images. In particular website filtering is effective only after the fact and does not prevent the creation of illegal material nor, in the case of images of child sexual abuse, the exploitation of children.

Doesn’t Catch the People Doing It

The system also will not remove illegal content from its location on the Internet, nor prosecute the creators or intentional consumers of this material.

Can Easily be Circumvented

The Department also acknowledges that website filtering systems are not 100% effective in preventing access to illegal material. A person with a reasonable level of technical skill can use tools that are freely available on the Internet to get around the filters.

Doesn’t Stop File Sharing or Chatrooms

As illegal material, such as child sexual abuse images, is most often traded on peer-to-peer networks or chatrooms, which will not be filtered, the Censorship Compliance Unit carries out active investigations in those spaces.

Might Give Parents a False Sense of Security

The Department is aware that a website filter could give parents a false sense of security regarding their children’s online experience. Filters are unable to address all online risks, such as cyber-bullying, online sexual predators, viruses, or the theft of personal information.

Maybe the DIA should persuade themselves that Internet filtering is a good idea before trying to implement it.