Former minister Alan Tudge has hit back at questioning at the robodebt royal commission suggesting he was was indifferent to the legality of the scheme.

Former human services minister Alan Tudge resumed his testimony at the robodebt royal commission on Thursday, after being on the witness stand all day on Wednesday.

Mr Tudge said he did not consider whether there was a 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 responded that he denied he had such an approach.

“My rationale was multifaceted in terms of why it had not crossed by 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.”

Mr Tudge’s fellow former minister Christian Porter will also deliver 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.

– AAP