Once you have been accepted on the Au Pair France program, you will need to apply for a visa to live and work in France. Information about applying for your visa can be found on the website of the Consulate-General of France in Australia (Sydney).
Level 26, St Martins Tower
31 Market Street
Sydney NSW 2000
Tel: (02) 9268 24000
You will need to click through to their Visa section and follow the instructions relevant to the Au Pair visa. (http://www.ambafrance-au.org/france_australie/spip.php?article1106&var_recherche=Au%20Pair)
Note that you will need to make an on-line booking to commence and progress your visa application with the Consulate-General of France. Please allow 4 weeks for processing of your visa application.