From au website, you could check of the availability for the sim unlocking of au mobile phones and will be able to begin the next step (No processing fee).
Tap "Proceed to unlock the SIM（My au)"
Choose the phone number you wish to process and tap「Select」.
The au phone number selection screen will appear only when integrated au ID.
Based on the integration, select from the au phone number associated with an au ID.
Enter the 4 digit security PIN code you have created when you sign up the contract with us, then tap "Next".
Security PIN code is the 4-digit number you have created when you sign up the contract with us.
If the PIN Code is unknown, please refer to the document provided as a copy of your contract, visit an au shop to change the PIN code, or call us and we will post it by mail (approximately one week).
List of the devices with valid purchase history will be displayed.
Eligible devices for unlocking will be shown.
For confirmation purpose only, the procedure ends here.
To proceed to unlock the SIM, please tick a device you wish to unlock and press "Next".
Select "reason for unlocking", and tap "apply with these informations".
Please refer to the below list "reason for unlocking".
SIM unlocking procedure has been completed!