This is an automated system for recovering debts, notably the one used by Centrelink to recover money overpaid to welfare recipients. The robodebt system relies on information produced by matching information the recipients provide on their income against other data to identify welfare fraud. It then sends a letter of demand to the recipient. It is up to the recipient, then, to prove their case.
The robodebt system operated largely unnoticed for a couple of years, but in that time it produced enough erroneous demands for repayment and enough additional stress on vulnerable people to cause an uproar when it did finally come to the attention of the public.