Terry Carney worked at the Administrative Appeals Tribunal, which appeals decisions made under Commonwealth law, for almost 40 years.

Long before the former coalition government dismantled the program, he knew something was wrong.

Carney repeatedly found debts calculated under the scheme lacked enough evidence and could not be legally enforced.

In 2017, he found against the program five times. Months later, his contract with the tribunal was not renewed.

Robodebt used a controversial data-matching technique to calculate debts on welfare recipients, comparing the income they declared to Centrelink with tax office records.

Carney ruled it was not the responsibility of welfare recipients to provide pay-slip data or risk being hit with a debt, adding that using an income average to calculate debts was illegal.

He found the practice of averaging income lacked “sufficient strength of evidence” and “simple mathematics”.

The commission will also hear from Barbara Martin, a pseudonym given to a Services Australia employee.

She will give evidence about how her organisation handled robodebt complaints.

Former department director Anthony Barford will also appear.

The royal commission this week heard from 76-year-old Rosemary Gay, an aged pensioner who was hit with an inaccurate $65,000 Centrelink debt and given less than a month to pay it.

After a dispute, the debt was scrapped following a four-year ordeal.

“It turned my life upside down, it was just sheer terror that I owed a figure that was just such a huge amount,” Gay told the royal commission.

“The fact I was to come up with that within a matter of three or four weeks, it was just sheer terror to me and I had no idea what to do next.”

Public hearings over the next fortnight will focus on the impacts of robodebt, along with the criticism of the scheme after it was implemented.

Former ministers Christian Porter and Alan Tudge are due to give evidence next week.

-AAP