New Zealand's Department of Internal Affairs has a "filter" that is intended to block access to pedophilic content.

It's currently optional -- meaning your ISP can choose to use it. I'm on cable and every single cable provider has opted in, so it's barely optional at all unless I switch to inferior more expensive DSL technology... but that's irrelevant as I have no doubt the end game is to make it compulsory.

What's the problem then?

1) This makes it trivial for future governments, or even public servants with a crusade, to start blocking / tracking other things. The Australian version is already blocking abortion info, and even a dentist's website.

2) it's trivial to circumvent for those that spend the time

I'm already needing to circumvent this filter - i need to connect to servers directly, not via a filter, in order to do my job. Also, at times the filter is unreliable and I can't connect.

Here's a simplified version of how these things work

Picture one: a normal internet connection, your computer talks to servers that contain the website you want to view. Website normally appear on port 80 (hence the :80 in the picture)

Picture two: The DIA filter in the way. Because websites are normally on port 80, your isp just redirects all port 80 traffic from your computer to a "transparent proxy" which then implements the filter.

Picture three: You reconfigure youre browser to not use port80. Instead it goes to a ultra cheap vserver you bought for a meesly $10, running in USA or russia or somewhere. It's no longer using port 80, so the filter doesn't appy.

Other ways to get around this: The website uses SSL - this is the same technology used when acessing internet banking. It's encrypted. It cannot be (easily) read by DIA, so no filter is applied
Another way is for the website to run on any port other than 80.

It's so trivial -- the DIA's filter is ludicous. It's not going to stop any but the stupidest of criminals, it's has very large potential for abuse by governments, and adds one more point of failure in your internet access.

Alas this is something very hard to talk about - because it involves an attempt to catch pedophiles - the worst scum in the universe - it is difficult for various techies who understand this to come out explaining why it doesn't work. This hampers the protest against this. Please think beyond that and consider how little this filter will actually do to stop actual criminals.