Struggling student

I first received a online in August letter saying that I needed to confirm my income.
The lady on the phone stated that I owed $7,000 in debt as I had under reported my earning. The lady also went on to make me feel like a criminal by going on how this was wrong and how did I intend to pay this back.

This all happened whilst I sitting my final exams for my Master of Physiotherapy.

I called up and tried to dispute this as the reporting that I did matched up with the ATO so I was confused.
I stated that I didn’t underreport due to this and they agreed to have another look for me.
Still nothing —I still owed $7,000 according to them.

I put in two written complaints with no reply, only a letter in November stating that I still owe this money.
I called the complaints team on the 17th of December and they were putting in an appeal for me.

Then I received a letter on the 22nd of December stating my new start allowance had $85.12 taken from my payment without my consent or knowledge.

In my written complaint I asked if all debts could be placed on hold due to myself completing my last lot of studies.
The only thing that they said was that centerlink had over paid me. That was all

Since August I have had increased amounts of anxiety and stress over this along with decreased grades.
How can they do this to a student who’s only income is centerlink? Who is trying to better themselves to help the community?
Right around Christmas time. Four months of anxiety and stress for something that’s illegal.