Ex-Department of Human Services deputy general counsel of program advice and privacy Mark Gladman told an inquiry he had raised concerns about the scheme in 2017.

Gladman advised former general counsel of commercial law Lisa Carmody that the argument in support of the proposal was “weak”.

“I believe I met with Ms Carmody to discuss the progress of the Averaging Advice late morning on 10 January 2017 (10 January 2017 Meeting),” Gladman wrote in an email shown at the robodebt royal commission in Brisbane on Monday.

“In that discussion, I told Ms Carmody that I thought the arguments that had been identified in support of income averaging were “weak”.”

Gladman urged the department to seek urgent legal advice from the Australian Government Solicitor.

“This is a high profile issue, being considered at the highest levels in DHS,” he acknowledged.

Gladman said the more he examined the details of the robodebt scheme, the more questions it raised.

“The more I looked at our arguments, the more I started to question whether that was really an accurate description of what the department was trying to achieve,” he told the inquiry.

He questioned how the calculations could possibly apply to students who worked sporadically at best.

Carmody said she did not recall Gladman raising legal concerns.

“I don’t have a specific recollection of Mr Gladman saying the arguments were weak,” Carmody told the inquiry.

“But from my review of the correspondence that week and my review of the draft advice, it is apparent to me that we weren’t expressing it in terms that it was strong.”

Commissioner Catherine Holmes criticised the legal argument used by the department to justify the averaging scheme, saying there was no justification under the Social Security Act.

“To put it bluntly, this is about as lame and advice as you could get, isn’t it? … as a legal advice, it’s really unconvincing,” the commissioner asked.

Carmody conceded it was unclear, and in hindsight it would have been prudent to get some external legal advice to support it.

The robodebt scheme continued to operate for several years despite concerns it was unlawful, with some people taking their own lives while being pursued for debt.

Former human services minister Alan Tudge has already fronted the inquiry, where he said his understanding of income averaging was that it had been used for decades and it did not occur to him it may have been unlawful.

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

Tudge has since announced his resignation from federal politics, triggering a by-election in his outer-eastern Melbourne seat of Aston.

The chief of staff and a former policy adviser to Tudge will front the inquiry in Brisbane at the fourth block of hearings into the automated debt assessment and recovery program.

Three other former social service ministers are also set to front the hearing this week.

Stuart Robert and Michael Keenan will front hearings for the first time while Marise Payne will reappear after previously giving evidence in December.

Annette Musolino, former chief counsel at the Department of Human Services, will also appear for the second time.

The royal commission is set to hand down its report on June 30 after the deadline was extended when an extra 100,000 documents were produced.

-with AAP