A key critic of Robodebt has told the royal commission the controversial scheme's central method lacked "simple mathematics".
Emeritus Professor Terry Carney told the inquiry he was sceptical about the then-Coalition government's now-defunct debt recovery scheme from the "get-go".
Professor Carney worked at the Administrative Appeals Tribunal (AAT) — which appeals decisions made under Commonwealth law, including Robodebt — for almost 40 years.
"From the very early stages, from the get-go so to speak … when I first became aware of budget papers, what was being proposed, all sorts of alarm bells went off in my mind that this was not a legally-founded process," he said.
"I thought I must be missing something, the legal foundation was, unquestionably to my mind, completely missing."
The 2015 program used data-matching algorithms to work out whether Centrelink recipients had been overpaid but unlawfully claimed almost $2 billion in payments from 433,000 people.
'Insufficient strength of evidence'
Professor Carney ruled Robodebt was illegal five times between April and September 2017, claiming its income-averaging method lacked "sufficient strength of evidence" and "simple mathematics".
The Department of Human Services (DHS) never appealed any of those rulings.
Counsel assisting the commission Justin Greggery asked Professor Carney whether it was "unprecedented for the department not to engage on a point of law".
Professor Carney said it was.
He said it was common that the DHS "rapidly appealed or went to the Federal Court or both" when a major policy initiative "was an issue".
In his written statement, he said the fact that the department never appealed was "the main reason it [Robodebt] was able to continue to operate for so long and at such cost to applicants".
"Had there been public ventilation of what the AAT was ruling … there wouldn't have been an instant change to or abandonment of the scheme but it would have been a lot quicker than the three or four years that nearly half a million people had to suffer the unlawful raising of debts," he told the commission.
The commission heard his bid to renew his contract as an AAT member was rejected by Cabinet in September 2017, the same month he delivered his fifth ruling against the scheme.
He said that "came as a surprise".
After giving evidence, Professor Carney told the ABC that if the department had appealed and lost, the decisions would have been made public and could have halted the scheme.
"If my decisions had been acted on by the department, we might have had only a few hundred people who were subjected to the unlawful decision-making of Robodebt," he said.
Asked if the DHS knew it would have lost an appeal, Professor Carney said "that's my speculation".
The latest hearings are looking at decisions by government departments and ministers.
The former Coalition government continued to defend the scheme despite growing criticism from tribunals and media in late 2016 and early 2017.
Former Liberal ministers Christian Porter and Alan Tudge will give evidence next week.
