These are some useful links about the proposed Mandatory ISP Filter.
Australia
Electronic Frontiers Australia http://www.efa.org.au/category/censorship/mandatory-isp-filtering/
EFA Open Internet http://openinternet.com.au/
Get Up! https://www.getup.org.au/campaign/SaveTheNet&id=684
Somebody Think Of The Children http://www.somebodythinkofthechildren.com/
Global
Electronic Frontier Foundation http://www.eff.org/
Index on Censorship http://www.indexoncensorship.org/
If you want to contact your local member or Senator Conroy, look here
House of Representatives http://www.aph.gov.au/House/members/index.htm
Senate http://www.aph.gov.au/Senate/senators/index.htm
EFA and Get Up! have some sample letters to get you started.