In order to change her name she either needs both parents consent or an Order from the Court. She shouldn't be appearing in Court herself though - that's for you to do on her behalf. Children are not generally welcome at Family Court hearings.

I'd suggest visiting a family lawyer (SA Legal Aid should give free advice) to get the forms and start the process.

She can always use her preferred surname at school, etc, but it won't be formally changed without a Court Order. Otherwise she can wait until she's 18 and do it herself.