The problem that Australians have with the extremely unpopular internet filtering proposed is that it doesn't block access to "sites featuring material such as rape, drug use, bestiality and child sex abuse." It blocks access to "unwanted content." The definition of that is completely left up to the government. There is no judicial oversight. There is no process for appealing a listing. The blacklist is completely secret and leaked copies have shown that its content is in no way limited to the above. It might be that the content initially may be deplorable, but the way the whole system has been put forward will easily allow them to escalate what is 'unacceptable'.
A secretive, government-mandated, government-run censorship system is a very, very bad thing.
However Anonymous has essentially shot those protesting the proposal in the kneecaps with this move. Now, in addition to claiming that anyone who protests the filtering system must be a pervert and a kiddy fiddler, the proponents can accuse us of being 'internet terrorists' or something as well.