He has provided pro bono advice and helped prepare correspondence to the department, which repeatedly asked for an explanation on how the debt was calculated. However, the department's compliance branch has ignored nine letters between May and November 2018 that requested additional information. Last week, it made threats to impose interest charges on the original debt. "Other than the bald assertion that I have a debt, I have never received any details of how the debt is alleged to have arisen or anything which would enable me to verify or understand the demand made of me," Mr Silbert's client wrote on June 7. In another letter, Mr Silbert's client wrote: "There is not a court in the country that will uphold your demands for interest in the absence of fundamental details of how the amount is alleged to have arisen." The dispute escalated further when the department engaged debt collection agency Dun & Bradstreet, which threatened Mr Silbert's client with a "departure prohibition order" that would prevent him travelling overseas.

Mr Silbert is keen to launch Federal Court action to test the legal basis of the robo-debt program and the government's apparent unwillingness to provide particulars. "I'm itching to get this before a court," he told Fairfax Media. He said legislation that regulates data-matching technology requires the department to "give particulars of the information and the proposed action" before it can recover overpayments. The robo-debt program, introduced by the Coalition government, calculates a former welfare recipient's debt by taking a fortnightly average rather than discovering the exact amount that was claimed. The department was forced to concede it was no longer in possession of the original claims made to Centrelink by Mr Silbert's friend, after he made requests under freedom-of-information laws.

Department of Human Services general manager Hank Jongen insisted that all its compliance activities were consistent with relevant legislation. Mr Jongen said a review by the Commonwealth Ombudsman had also found the method of calculation complied with legislation. "Of the thousands of DHS payment cases that go the Administrative Appeals Tribunal, the vast majority of decisions made by the department are affirmed," he said. "When decisions are set aside or varied it's almost always because new evidence has been provided by the customer or obtained by the department." But earlier this year, Professor Terry Carney from the University of Sydney released a scathing review of the program, which he accused of wreaking "legal and moral injustice".

A member of the Administrative Appeals Tribunal for 40 years, Professor Carney accused Centrelink of failing to defend the legality of the debts in the AAT, where several welfare recipients had made appeals. He said their conduct "arguably breaches" the model litigant obligations of a government agency. He also criticised the income-averaging method used by Centrelink to calculate debts, despite a report from the Commonwealth ombudsman in 2017 that found the method caused debts to be "greatly inflated". Professor Carney said the "failure of a person to 'disprove' the possibility of a debt is not a legal foundation for a debt". "When confronted with suggestions of having an overpayment, often up to seven years ago, the least literate, least powerful, and most vulnerable alleged debtors will simply throw up their hands, assume Centrelink knows that there really is a debt, and seek to pay it off as quickly as possible," Professor Carney wrote in the UNSW Law Journal published in March.