Former minister Marise Payne has told a royal commission into robodebt she doesn’t know how early concerns over the scheme’s legality were left out of later policy documents.

Senator Payne was human services minister at the time the program was formulated, working underneath then social services minister Scott Morrison.

She appeared as a witness before the commission in Brisbane on Tuesday.

Early ministerial briefings from the Department of Human Services about the scheme suggested using Australian Taxation Office data as an alternative to the “cumbersome” method of cross-checking income declared by customers.

The briefings suggested “income averaging” as a way of arriving at fortnightly rates from annual tax data.

It was laid out in drafts about the proposed changes viewed by Senator Payne that “some of the options … would need legislative and/or policy changes”.

It was also noted advice had been sought from the Department of Social Services which advised “some proposals will come under significant scrutiny as not being consistent with the overall beneficial nature of Social Security law”.

Those legal concerns were not reflected in later government policy proposals, resulting in the scheme being implemented regardless, with ultimately disastrous results.

More than 381,000 people were wrongly pursued for welfare debts totalling more than $750 million, with several victims taking their lives due to the distress it caused.

Senior counsel assisting Justin Greggery KC asked Senator Payne: “How did the identification of the problem … drop off the radar by the time it seems the (expenditure review committee) considered the proposal?”

“I don’t know the answer to that question and I say that in all transparency,” she replied.

“There is an expectation, however, that when agencies advise you, on the face of a brief, that they are working together to address these issues.”

When asked who held responsibility for advancing the concerns, Senator Payne said ministers were always “ultimately responsible” but must receive advice from their departments.

Commissioner Catherine Holmes put to Senator Payne if, in viewing the draft briefings, she would not have asked what the possible legislative changes were.

“I think at the stage at which the process was … (I would) not necessarily have asked for specifics at that point, no,” she replied.

ATO director of data management Tyson Fawcett, who has been with the tax office for more than 30 years, told the commission on Monday averaging annual income data to arrive at a fortnightly rate is simply not an accurate method.

Last week, former Services Australia general manager of business integrity Mark Withnell rejected a statement the controversial income averaging process, also known as “income smoothing”, was fundamental to proposals for the scheme.

Mr Withnell denied memory of a 2015 meeting at which it has been claimed he was frustrated when told an income averaging debt calculation practice was not lawful.

Another former public servant, ex-Social Services employee Catherine Halbert, denied in her evidence manipulating the truth about the department’s view on the scheme in 2015, insisting it did not endorse income smoothing.

Meanwhile, former Human Services secretary Kathryn Campbell told the commission she knew law changes were needed in 2015 to use an average income instead of actual income for debt calculation, but said Social Services was in charge of the matter rather than her department.

– AAP