Centrelink Overpayment

Can Centrelink debt be waived if you were very ill and were forced to drop your study load? I was studying full time up until then and had to come down to part-time. I didn't know I wasn't allowed to be part-time with youth allowance so I didn't report it then but when I found out I immediately got a medical certificate and uploaded it and I also uploaded my study load and it was approved so I just assumed they were okay with it. Now I just got a call saying that I will need to pay everything back for that semester.

Any advice on what I can do?