I did this when I was about your DD's age also. I think to start with Mum just wrote a letter to my high school saying I wanted to be known by mums maiden name from now on. So basically from the age 12/13 till I was 16, I was known as "mums maiden name" Then when it came time for my licence I had it done properly. I had already been suing it for a long time, I had my TFN as that name also. So really all I had to do was fill in a form & take it to the registry & pay the fee. I didn't have a birth certificate only a birth extract so when I did all this I got an official birth certificate also & it has my birth name as my mums maiden name. Doesn't have my dads surname on it all . this was also in NSW.

I would start with changing her name at school & just leave it at that until she starts to need official documents when she gets a little older. Mum wrote something like the want of the name change was to do with the breakdown of the marriage between my Mum & Dad. Even though that happened when I was 6 months old.

eta - at no point was my dad contacted/informed or needed for me to change it.