Am so sorry to hear that - must be pretty horrible. While I obviously can't comment on your case, if it was produced by averaging income then there have been recent legal precedents that have changed things a little.
The National Social Security Rights Network and the Welfare Rights Centre seem on to this. I don't know them, but if your debt is still outstanding maybe check them out and contact?
Here are some links that explain things a bit more:
https://www.9news.com.au/national/centrelink-robodebt-to-be-halted-immediately-debts-frozen/c9e2dc06-efda-4e58-a9d2-cdd15a43c856
Older stuff about why some robodebts are illegal:
https://www.theguardian.com/australia-news/2018/apr/04/centrelink-robo-debt-program-accused-of-enforcing-illegal-debts
https://www.theguardian.com/australia-news/2017/apr/10/centrelink-debt-scandal-report-reveals-multiple-failures-in-welfare-system
http://www.unswlawjournal.unsw.edu.au/wp-content/uploads/2018/03/006-Carney.pdf
https://www.theguardian.com/australia-news/2017/jun/21/senate-inquiry-calls-for-centrelink-robo-debt-system-to-be-suspended-until-fixed
Best of luck.
