Maybe your question is answered here already:
No. As both of your parents had the same surname, you have received their “married name” as your birth name and a name declaration is not needed.
You might, however, need a name declaration if you have changed your name since then (e.g. through marriage):
Yes - except if you were born before the 01.09.1986. If you were born before the 01.09.1986, Australian law might be applicable, in which case a name declaration will not be needed.
If, however, you were born on or after the 01.09.1986 and/or German law is applicable, a name declaration will be needed to determine your first name. You can chose between your father's or your mother's name.
No. According to German name law, a child of an unmarried mother automatically receives the mother's name. Therefore, a name declaration is not needed.
According to German name law, a child of an unmarried mother automatically receives the mother's name. This can only be changed as long as the child is younger than 18 years. Once the child has turned 18, a name change is not possible under the regulations of the Civil Code (BGB).
It might be possible to change the name under public law, however, this is difficult and not always possible.