About a year ago I also received a Robodebt notice that I owed about $5.5k in total from two financial years, from 2014 to 2016. I had lodged my tax return and received my FTB as a lump sum on both of those years. Four phone calls to Centrelink later, I was told that they had made a coding error and they would sort it out for me. Two days later I received a letter stating that (instead of owing a debt of five and a half grand) that I owed them a balance of $0.00. A good result, you would think, except that this financial year exactly the same thing happened, resulting in an alleged debt of $1,120. When I rang to explain that they must have made the same mistake again, the operator told me that the system was far too complicated for her to understand and that she would have somebody call me back who could better explain the reason for the debt. Not. Good. Enough. My response was to upload a form requesting a 'Review of decision' to my MyGov account a couple of weeks ago but I doubt they'll bother following that up in a timely manner. They no doubt want to grab part of my tax refund in the meantime, rather than deal with their own (repeated) error.
Centrelink are great at generating erroneous debts but terrible at responding to requests to fix up their own mistakes.