I've had many, uh, "fun" dealings with whatever name Centrelink are going by this time. They tried to slap me with a robodebt, picking a financial year where for the first six months of the year, I was away from home at university, was recieving a payment and only worked four weeks (during uni breaks). Once uni finished, I went back home and worked full time and stopped receiving the payment.
Of course, they averaged out the income from all 7 months I was working over the 12 months, and reached the conclusion that I was some kind of welfare cheat. I was able to get my payslips from my employers, so off I toddled to their website, where I was presented with a form that split that year into their fortnights. Their fortnights started and ended on a Wednesday, so I had to sort my pay information into their buckets because I wasn't paid fortnightly on a Wednesday.
Naturally, once I'd done this and submitted the form (no more than 15 minutes) it timed out, so start again - fortunately I had the good sense to keep the spreadsheet with the info in it.
In a move that should surprise precisely nobody with more than two functioning braincells, no debt existed.
In this case I was lucky in that I had the resources, time and ability to get the payslips, sort the information out and send it to them and that my employers still existed so they could retrieve that information. Not everybody had this, particularly when they got one of the first versions of the letter that said "by the way, you owe us $10000 plus 10% for being a dirty welfare cheat" years after the fact.
It also didn't help that they themselves frequently couldn't calculate the debt properly. Here's an article where one person went from a debt of just over $1300 to a debt of nothing: https://www.themonthly.com.au/issue/2019/august/1564581600/robert-skinner/how-i-fought-robodebt - tl;dr, after several recalculations, each spitting out a new supposed debt (including one where they owed him money), they reached the conclusion that no debt existed.
This was purely an ideological system, solely to punish those who dared to be poor or unemployed and needed help from the government. I don't believe they ever got more back from it than they ever spent on it even before this, and if they don't settle with the class action for compensation, I expect there's going to be some very dirty laundry come out as a result.