Utterly confused

I received an email in my junk email to say I had a myGov email. The email asked me to confirm my earnings from 2010-2012. I checked my tax from those years and the amount Centrelink said was the same as I had on record so I confirmed amounts as correct...then up pops a $5000 debt! I was confused as this totalled more than I had received from Centrelink during that period. I was working as a relief teacher at the time so was only on a Newstart payment for two months over the Christmas break in 2010-2011 period then stopped reporting once I started getting casual teaching work. During 2011-2012 period Centrelink insisted that I keep reporting for 12 consecutive weeks...I trusted what the Centrelink staff had told me and reported (mostly estimating my income) over that time. I was always careful to report more than I thought I would earn. To get this debt after so many years was a total shock! I checked the 'call back' button and waited for a response. I waited a week and no call back so I called compliance to find out what had gone wrong...I was treated like a criminal...comments like "what you have done wrong" by the person I was speaking to. They kept giving me scenarios that did not apply to me and after 45 mins of trying to get answers I was still utterly confused. It took many more calls to them to get answers to what I needed to do to clear my debt.
I then frantically rang all the schools I had worked for during these periods and requested pay slips. Department of Education were great and emailed all my payslips by end of that day. Other independent schools had changed computer systems and no longer had my payslips. Some schools were able to offer spread sheets with pay amounts and statement of services. I then spend hours/days going through all my paperwork and luckily found 6 year old payslips (some missing). Legally, you only have to keep personal evidence for 5 years, so how Centrelink can go back 6 years is beyond me.
After scanning and uploading (another 5-6 hours when I should have been writing class reports) all the documents I thought I was safe. Three weeks later I had a missed call on my mobile from Centrelink, a no number call so I assumed Centrelink. Then a message from front office from school that Human Services needed to speak to me desperately. I couldn't believe they'd called my work place...!!! I called them back and they informed me my debt was going to collection unless I made payment arrangements. OMG! I explained that a review was in place but they still insisted I start paying. I made arrangements for $100 per fortnight to be paid.
After many more calls and a 5 week wait I finally got notice that my 2010-11 debt was cleared but I still owed $1300 on my 2011-12 debt..their workings say I under estimated $77 in that year??? Go figure... Also, even though I made payment arrangements and have tightly stuck to them they still sent me a late notice and demanded full payment. I once again called them and after 30mins of explaining and been made feel like a fraud he finally apologised and said I should not have been sent the late notice.
I've been too exhausted and scared to do my last tax return in case this happens again, but I know I must and when I do I'll get my accountant to go over my 2011-12 figures just to be sure. If I need to I will keep fighting.