Legislation to enable mandatory censorship legislation will hit Parliament next month. To highlight the issue during the week of Australian Day, Electronic Frontiers Australia is backing a 'blackout' campaign encouraging people to darken their social networking profile pictures and web sites.
While online protests can seem like an example of 'slacktivism', using the Internet seems appropriate given that it's the very medium that the legislation would restrict (and restrict in ways that we wouldn't be allowed to know about). Darkening your avatar also provides an opportunity to explain the issue to friends if they ask "Hey, what's up with your picture?" If you're not sure how to go about darkening your picture, there's detailed instructions on the site.
One aim of the online protest is to encourage people to sign the EFA treaty opposing the move, which will eventually be presented to the Senate. Don't forget all the other ways you can signal opposition to the plan too (writing to your MP remains probably the single most valuable thing you can do).