(Cross-posted from a comment on Facebook)
In reply to a comment that Christians should back the proposed internet filter because opposition to porn is a Christian value, I wrote:
1) Backing the filter isn't opposing porn, it's opposing one method of obtaining porn, and opposing it in a way that's contrary to Christian values - end-justifies-the-means reasoning.
2) A web-based filter like the one currently proposed isn't going to work, anyone who's looking for it will very easily get around it (even non-computer literate people can easily use peer-to-peer file sharing, which won't be filtered).
3) The internet is by its nature almost impossible to censor - both URL-based and content-based filters can be bypassed with very little effort.
4) Once the government have the setup to ban porn they can easily ban anything else that suits them. We already have Racial and Religious Discrimination legislation which makes it potentially illegal to preach the Gospel. Any church web site which states that Jesus is the only way to heaven might find itself on the blacklist.
5) False positives (and false negatives) are almost guaranteed on any data pool as large as the internet. Judging from the length of time between versions of the blacklists (as publicly known) and the number of legitimate URLs included, corrections will take days, weeks or even months to be actioned.
6) Filtering will slow down internet access by a significant margin. (Don't believe what Senator Conroy says on the matter, he has proven many times that his knowledge of even simple matters is sketchy to say the least.)
7) The cost of maintaining the filter (eg paying a department full of people to process requests for blocking and appeals against false positives) will be huge, and naturally grow at the same pace as the internet grows.
8) A simple glance at the publicly available blacklist shows that the people operating it have a very poor knowledge of the way the web works, or else very little regard for either false positives or false negatives.
Filtering to combat porn is like swatting a fly with a wok - a lot of effort, a lot of collateral damage, and a high chance that the fly will survive due to the bowl shape of the weapon.