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.