Former cabinet minister Alan Tudge has told a royal commission he was not responsible for his department's failure to ensure the Robodebt scheme was lawful. 

Mr Tudge, who was human services minister in the Turnbull government between 2016 and 2017, is the third ex-Coalition government minister to give evidence at the royal commission into the unlawful Centrelink debt recovery program.

He was in charge of the scheme when the lawfulness and accuracy of its central method, "income averaging", was first questioned in the media as Robodebt recipients who were being pursued for false debts came forward.

The now-shadow cabinet minister repeatedly defended the program, which continued to operate until 2019 and wrongfully accused hundreds of thousands of welfare recipients of owing money to Centrelink.

Amid growing criticism and negative publicity, Mr Tudge told the commission he relied on knowledge that "income averaging" had been used for decades by "successive governments" and that it was the responsibility of department officials to confirm its legal basis.

Counsel assisting Justin Greggery KC put to Mr Tudge that it would have been a "simple thing to ask for advice" to answer questions raised about whether the scheme was lawful.

"This was a program which had been through a cabinet process, and … a cabinet process is a rigorous process which always has a legal overlay through it," Mr Tudge said.

“So the Department of Social Services' lawyers would have had to form a view that it was lawful … the Attorney-General's Department has to form a view in relation to the legalities of this initiative.

"I also became aware, I think at that stage, that income averaging … had been used … as far back as 1990. It had been used for decades under successive governments.

"My focus was very much on … the administration of the scheme and how should that be addressed."

Mr Tudge added that the department secretary had a legal obligation to raise "significant" legal issues with him.

"It's unfathomable for a secretary to be implementing a program which he or she would know to be unlawful," Mr Tudge told the commission.

"I would not expect a secretary to raise with me, 'Hey … there's question that this scheme might not be lawful,' … she's going to solve that question before raising it with the minister," he said.

'I don't know this silk'

Mr Tudge was then asked about a 2017 national law conference attended by several officials from the Department of Human Services (DHS), including the secretary and chief counsel, in which eminent silk Peter Hanks QC criticised the Commonwealth's method of raising and recovering debts in a keynote speech.

"What do you expect the secretary and chief counsel to [then] do?" Mr Greggery asked.

"My expectation is that it would make them inevitably contemplate what the [legal] advice was that was given and reassure themselves that what they were doing was still lawful," Mr Tudge replied.

Mr Tudge then said he did not know who Mr Hanks was.

"I don't know this silk," he said.

Commissioner Catherine Holmes interjected: "You didn't study … Hanks Constitutional Law at law school? It's the only book I had."

"You're testing my memory now, Commissioner," Mr Tudge replied.

Commissioner Holmes said: "I think I probably have to remember further back than you, Mr Tudge, and I can remember that textbook."

"I don't recall … I did study constitutional law. I simply don't know the name," Mr Tudge said.

"… Peter Hanks QC. I don't think I know that name, sorry. Perhaps I should know and I apologise to Peter Hanks if I should know who he is."

Mr Tudge went on to say it was his expectation that the DHS secretary and chief counsel would refer the speech to department lawyers.

'I was responsible for the implementation of the scheme'

Mr Greggery put to Mr Tudge that if it was a "failure" of the secretary and chief counsel not to take that step, it was "a failure for which you are responsible as minister".

Mr Tudge replied: "I don't know that you can say that."

"But you understand the concept of ministerial responsibility?" Mr Greggery asked.

"In the broad scheme … yes, but to say, the way that you put it, that I was responsible … I don't think is right," Mr Tudge said.

"Surely that falls under the broad umbrella of ministerial responsibility, where when there is a shortcoming in the department, the minister is ultimately responsible for that?" Mr Greggery continued.

"I accept and understand deeply the Westminster concept of ministerial responsibility … I don't accept the proposition that I was responsible for an individual not making a choice to not raise a matter," Mr Tudge said.

"I was responsible for the implementation of the scheme."

He later accepted Mr Greggery's proposition that he was also responsible for the program's "lawful implementation".

The commissioner asked Mr Tudge: "You could see, because it's not that hard to see, the potential for problems with averaging to produce inaccurate results. Did you wonder at all how it was permissible under the act?"

Mr Tudge replied: "That did not cross my mind."