Former Liberal minister Alan Tudge has denied responsibility for the disgraced robodebt scheme being illegal.

Mr Tudge, who was human services minister between 2016 and 2017, told the royal commission into the Centrelink debt recovery scheme that while he was in charge for matters in his department, he wasn’t responsible for the scheme’s “lawful implementation”.

During his appearance at the commission on Wednesday, Mr Tudge said while he did not consider the legality of the robodebt scheme, questions of the program’s fairness were brought up.

“It is unfathomable for a [department] secretary to be implementing a program which he or she would know to be unlawful, it is unfathomable,” Mr Tudge told the commission.

The scheme used averaging on incomes to determine debt levels and ran from 2015 to 2019, continuing to operate despite concerns over its legality.

The controversial program recovered more than $750 million from more than 380,000 people. But it also led to several taking their own lives while being pursued for false debts.

Mr Tudge said he had no recollection of being “excited” about the prospect of recovering money as part of data-matching activities, despite emails between tax office staff presented to the commission stating he was.

Mr Tudge also told the commission he could not remember looking to extend the time period officials could use to recover debts.

Concerns were raised that the time period was looked to be stretched back to as far as 2000.

The commission was told that in the time shortly after becoming human services minister, then finance minister Mathias Cormann asked Mr Tudge if there was any way to find more budget savings in the portfolio.

Despite concerns being raised in late 2016 and early 2017 in the media about the legalities of the controversial scheme, Mr Tudge said he was focused on its implementation.

On Tuesday, the royal commission heard how Mr Tudge sought the Centrelink file details of robodebt recipients who had spoken out in the media about the scheme.

Mr Tudge’s former media adviser, Rachelle Miller, said he had “requested the file of every single person who appeared in the media so that we [could] see the exact transactions that they’d had with Centrelink and understand what the details of their case was”.

Former Liberal minister Christian Porter, who was in charge of social services at the time of the scheme, will also testify before the royal commission later this week.

– with AAP