The Morrison government has refused to release any legal advice on the botched Centrelink robodebt scheme to a Senate inquiry examining the program, prompting claims from Labor of a “whitewash”.

Guardian Australia has learned that the government services minister, Stuart Robert, has refused to provide the inquiry with any legal advice the government has ever sought about the legality of the program.

Citing public interest immunity, he has also declined to say when any advice may or may not have been provided or who it was prepared by, if indeed it exists.

Doubt swirls over the future of the program – including how many welfare recipients will be affected by the decision to review debts that were issued using “income averaging”, a much maligned process the government abandoned late last year under the shadow of a court challenge.

Labor’s government services spokesman, Bill Shorten, said the public deserved to know what led the government to believe that the “scheme was legally justified”.

“There are three possibilities. Either they got bad legal advice, they got good legal advice and ignored it or they recklessly commenced their program against innocent Australians without ever seeking legal advice as to its legality,” he said.

Shorten said Labor would “consider all options to fight this whitewash in the interest of giving the myriad victims of robodebt the answers they so justly deserve”.

“Hundreds of thousands of Australians have had more than a billion dollars taken from them in suspect debt claims by the inaccurate robodebt scheme,” he said.

In general, federal governments, including the previous Labor government, have refused to release legal advice, citing decades’ of historical precedent.

A spokesman for Robert said: “It has been the long-standing practice of successive Australian governments not to disclose legal advice.”

The Senate inquiry, chaired by the Greens Rachel Siewert, was established last year amid growing public anger at the scheme.

In November, the government settled a legal challenge brought by Victoria Legal Aid on behalf of a Melbourne woman. The court declared by consent that the $2,500 debt – which was issued using “income averaging” and eventually revised by Centrelink to less than $2 – was “not validly made”.

Days earlier, the government overhauled the scheme, saying it would only issue debts where it had gathered evidence of the income earned by welfare recipients rather than simply averaging a person’s annual income across 26 fortnights.

At the time, the attorney-general, Christian Porter, said the government would not waive privilege over any legal advice it had sought about the program.

But Porter appeared to confirm the government had sought legal advice in some form. “You could imagine that any government … seeks advice on those individual matters, and that’s fed in, obviously, to decision-making in the Department of Human Services and in the minister’s office,” he said.

Guardian Australia reported on Tuesday that nearly 10,000 people had joined the Gordon Legal class action demanding compensation over the scheme.