This could mean that someone has tried to guess the answers to your secret questions on the Password Station (passwords.aus.edu) in order to attempt to reset your password.
When the secret questions are answered wrongly 3 times, the user is unregistered from the Password Station and must re-register. This is done to stop anymore potential threats to your account.
In the case of receiving such emails, you must log onto the Password Station (or re-register if you had been unregistered) and change your secret questions & answers as well as the password.
Please refer to the following FAQ's for help on how to change your secret questions and how to change your account password;
How can I change my password?
How can I register on the Password Station?