Here are some petitions you can sign.

This is the official petition addressed to the House of Representatives:

http://www.wakinggiant.org/au_censorship.htm

These petitions are not official, but are also worth signing as an indication of how much opposition there is to this policy:

http://petitions.takingitglobal.org/oznetcensorship
http://www.PetitionOnline.com/filtarud/petition.html

but there are much better ways of helping out. Check our forums for ideas.