What’s the process if you want to change your name?
Given that you are over 18 you can do one of two things:
1. Simply assume the name you wish to be known by; or
2. Formally apply to change your name at the Registry of Births, Deaths and Marriages in your State or Territory.
Whilst option 1 is legal, you will have a lot of practical difficulties when you go to open a bank account, or apply for a passport, or drivers license. In fact, you will strike difficulty in any circumstance where you are required to provide proof of your identity.
With respect to option 2, you need to comply with the legal requirements of the State or Territory in which you were born or adopted. We recommend that you contact the particular Registry to find out what their requirements are.
NB. This answer is provided on the assumption you were born in Australia.