Stuart Robert and Michael Keenan will front hearings for the first time next week as the royal commissioner continues to grill public servants about who knew what about the illegal debt-collecting scheme.

Marise Payne will reappear, having previously given evidence in December.

The chief of staff and a former policy adviser to another former human services minister Alan Tudge will also front the inquiry next week.

On Wednesday, Commissioner Catherine Holmes told a bureaucrat that robodebt victims had an “ice cube’s chance in hell” of trying to have their debt claim overturned by Centrelink.

The commission, set to take evidence in Brisbane on Thursday, has been told  senior bureaucrats were aware of the potential illegality of the scheme, but were either overruled by the people in charge of deciding the department’s policy, or were too scared to come forward.

The commission heard evidence that the department wanted the debt of a plaintiff reassessed to end legal proceedings as soon as possible.

Counsel assisting Angus Scott suggested one reason to avoid a court determining the legality of the scheme was to ensure “there is no public revelation of the extent of unlawfulness”.

The department’s former acting chief counsel, Tim Ffrench,  said he understood the department’s legal position to be weak.

He said people genuinely believed the scheme was lawful, and there were “audible gasps” when he revealed advice saying the department’s court prospects were poor.

“There was no judicial authority to support a proposition that income averaging in a social security context was lawful,” he told the hearing on Wednesday.

Ffrench said he recalled then-minister Mr Robert telling him “it’s just an opinion until we get a judicial declaration from a court” when presented with advice from the government solicitor that the scheme was potentially illegal.

He said he raised the need to seek legal advice from the solicitor-general after seeing advice during a court case testing the validity of a welfare recipient’s debt notice.

Robert responded with a statement along the lines of an “opinion just being an opinion”, but added he didn’t recall the then minister disagreeing with him.

Ffrench said Robert made no indication that external legal advice needed to be sought in light of the revelation.

He said he overheard then attorney-general Christian Porter tell Robert the solicitor-general’s advice about the illegality was “right, mate”.

Ffrench also pointed to a culture that prevented people from coming forward, with department heads “determined to achieve a particular outcome for government”.

“I believe the culture and environment at that time prevented people from asking the questions that they should have asked, because of the fear that those questions would be seen as potentially impertinent,” he said.

The final hearing block, which continues until March 10, will examine how the illegal scheme was able to be implemented.

The royal commission is set to hand down its report on June 30.

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

-with AAP