I used to work for centrelink, and if you were in a relationship, obviously you will need to repay the money.
But, the most likely scenario will be that nothing will come of it. Centrelink does understand that things can happen... You are allowed to date someone... you are allowed to have relationships... just as long as the person you are with is not living with you, not paying your bills, and not providing you with other financial assistance, there really is nothing they can do. Yep, an investigation may jut happen... you might even have to provide the names and contact details of people who can prove that you're not together, and had nothing but a slip up. This is actually a fairly common situation... not one I had to deal with personally though.
You'll be fine... if you're really worried, ask to speak with the social worker, they will help you sort many things out ( the social workers have a different, private system from regular centrelink officers)
HTH