Centrelink fraud or giving Centrelink misleading or false information is a criminal act and taken quite seriously. I cannot advise you as to what you should do. However, read this fact sheet from the Victorian Legal Aid and perhaps speak with a lawyer from your local community legal centre as to your options. The Fitzroy Legal Centre is particularly good as they have drop-in sessions on most evenings without need for appointment.
Hi Dan, cases of centrelink fraud are generally prosecuted under s135.2(1) of the Cth Criminal Code, "obtaining a financial advantage from a Commonwealth entity" which carries maximum penalty of 12 months jail and/or a fine of $6,600. It is a summary offence. Even if you pay back the money that you took, you can still be charged with an offence.
I would seek personalised legal advice before you do anything else as Sarah said.