Former Coalition minister Christian Porter is facing questions over his role during the height of the Robodebt scandal as he made his anticipated debut at the royal commission.

Mr Porter was social services minister when the debt collection scheme was wrecking havoc on people’s lives in 2017. He held the portfolio from 2015 to 2017 before being promoted to attorney general.

Mr Porter took the witness stand on Thursday after the man who followed him in the role, Alan Tudge, hit back at questions suggesting he was indifferent to the legality of the scheme.

Mr Tudge resumed his testimony at the robodebt royal commission on Thursday after a full day on the witness stand on Wednesday.

Mr Tudge said he had not considered whether there was legal authority for robodebt notices to be sent out.

“My mind was not acting as a lawyer. It was acting as an implementer of the policy,” he told the commission.

“I’d understood that (income averaging) had always been used for decades and so it had not crossed my mind that it could possibly be unlawful.”

However, royal commissioner Catherine Holmes said the minister’s approach to the scheme was indifferent to the mounting criticisms, particularly on the legalities.

“It seems a fairly blithe approach for a minister, particularly in the light of controversy, to assume that because it’s happened before for a long time it must be fine,” she said.

Mr Tudge denied he had taken such an approach.

“My rationale was multifaceted in terms of why it had not crossed my mind that it would be unlawful,” he said.

“One of the pieces of evidence … was that it had been through a cabinet process, which I know is rigorous.”

Documents presented to the commission showed Mr Tudge’s chief of staff wrote to the Department of Human Services in January 2017 asking how much money the budget could save by having debt notices calculated manually, rather than through automated systems.

The department responded that there would be savings to the budget bottom line of $150 million if manual processes were still used but the savings would be $1.2 billion if automation was undertaken.

However, Mr Tudge said he did not know his state of mind at the time about possible budget savings.

Mr Tudge also detailed meeting then prime minister Malcolm Turnbull about issues with robodebt.

The former minister said while he had not provided briefings on whether the scheme was legal or not, problems with how robodebt was implemented were discussed.

Mr Tudge’s fellow former minister Christian Porter also began his testimony at the commission on Thursday.

Mr Porter held the social services portfolio between 2015 and 2017 and is the fourth coalition minister to appear.

The former social services minister said he first learnt of the robodebt scheme in April 2016 from a ministerial submission.

Mr Porter also served as acting human services minister in late 2016 and into early 2017, which coincided with mounting criticism of the scheme, while Mr Tudge was on leave.

He said he became frustrated with the lack of information provided to him by the department about the scheme.

He also said talking points provided to him which he used in media interviews during that time avoided the question of whether income averaging was used as a way to calculate debt.