The former human services minister 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 with 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 will also give evidence at the commission on Thursday.

Mr Porter, who held the social services portfolio between 2015 and 2017, will be the fourth coalition minister to appear.

The Centrelink debt recovery scheme operated between 2015 and 2019 but continued well after significant concerns were raised about its legality.

It recovered more than $750 million from more than 380,000 people and several people took their own lives while being pursued for false debts.

Mr Porter will be the first of two new witnesses to appear before the commission on Thursday, with former social services group manager Janean Richards also giving testimony.

Australian Associated Press