I'm no help. I never changed my name but if I were you I would just travel on the old passport and get a new one when it expires.
I could be wrong but I think that if you apply for a new passport within one year of your marriage with your new name on it it will be free for the duration of your old one ie if your passport expires in 2010 then it's free for the 2 years.
If you ring DFAT they can tell you the rules and regulations.