Former prime minister Malcolm Turnbull says he questioned whether the department responsible for overseeing robodebt was providing its minister with correct information about its operation.

But he never doubted the scheme’s legality because the proposal had originally been presented to cabinet as not requiring any legislation change.

Mr Turnbull, who was prime minister at the height of the debt recovery scheme, was the commission’s first witness on Monday in its final week of hearings.

The robodebt scheme ran from 2015 to 2019 and used income averaging of tax office data to calculate and raise debts.

Following negative media reporting about incorrect debts being raised in January 2017, Mr Turnbull told the commission he urged then human services minister Alan Tudge to conduct a “frank assessment” of what was occurring.

He said he was concerned about reports Centrelink was not ensuring people were receiving letters about potential debts.

In a WhatsApp message to Mr Tudge, the former prime minister asked: “Are you sure your department is giving you the right advice on what is happening?”

“I ran a traditional cabinet government … I was very clear that I expected ministers to take responsibility for their departments and portfolios,” Mr Turnbull said.

“I was pressing him (Tudge) to do his job.”

Mr Turnbull said he regarded Mr Tudge as a “technocrat” who had a lot of experience.

“I didn’t regard him as being a negligent or incompetent or careless minister,” he said.

The commission is examining how the scheme was allowed to continue, given significant concerns had been raised about its legality.

Mr Turnbull said his office was concerned about the accuracy and fairness of the scheme, but not the legality.

“Cabinet was told that it was lawful in the sense that it was consistent with the legislation,” he said.

“My concern was essentially accuracy and fairness … we didn’t turn our mind to legality or lawfulness because we’d assumed that was as it had been represented.”

But Commissioner Catherine Holmes said the original proposal that sought legal advice from the Australian Government Solicitor in 2014 did not reference income averaging.

“If averaging is the key issue then it may be that the AGS had not advised on that,” Mr Turnbull said.

The government unlawfully recovered more than $750 million from more than 380,000 people through robodebt with the debt notices being blamed for contributing to multiple suicides.

Other witnesses set to take the stand at the commission on Monday include former human services director Kristen Foster and Services Australia general manager Jason McNamara.

The commission will conclude its hearings on Friday, with the final report due to be handed down on June 30.

The deadline for the report was extended after an extra 100,000 documents were produced.

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– AAP