A royal commission to investigate what went wrong with the robodebt scandal is needed to bring justice and answers, the Greens have said.
The call comes as Social Services secretary Kathryn Campbell confirmed the government had made the decision to refund the false debts more than 24 hours before the announcement was made late on Friday afternoon.
The government will pay back $721 million to 373,000 people who had debts raised unlawfully through the online compliance initiative run by Centrelink from 2015 until 2019.
But since then Prime Minister Scott Morrison and his ministers have refused to apologise for the program, citing the class action still before the court.
"I can see no other way forward than through a royal commission ensuring a full, independent review of this program and a forensic audit of this mess", Greens Senator Rachel Siewert said.
Senator Siewert, who is the party's spokeswoman for family and community services, said a royal commission would mean that every debt was audited, even if it was raised before the 2015 date set by the government.
Attorney General Christian Porter has admitted that debts were raised before 2015 using income averaging with tax office data, meaning they also would be considered unlawful.
The government is only repaying debts raised since 2015 when an automated system was introduced to compare tax office data with reported income to Centrelink and raise debts were discrepancies were detected.
"I am sure that people who received debt notices before 2015 will come forward, they won't and I will not forget the pain and anguish caused by this illegal process," Senator Siewert said.
The Greens also responded to a report that the government has not ruled out introducing new legislation to allow it to continue to raise debts through the controversial system.
"The government has been heartless and cruel throughout this entire process and their failure to even consider apologising means we cannot trust them to not do it again," she said.
"It's absolutely beyond the pale that after all this unnecessary pain and suffering the government still hasn't ruled out introducing new laws to allow a future reboot of a robodebt type scheme."
The law firm responsible for the class action that forced the government's hand on the refunds has joined calls for an apology, and said a statement to that effect won't be used against the government in court.
"We are prepared as the lawyers acting for the applicants and the group members to not use the fact of an apology in court if the government is willing to give a proper and genuine apology," senior partner Peter Gordon said.
"I make that undertaking on behalf of the applicants and the group members because we believe that it is really important to the people who have been hurt so much through this to hear the government say, not just that it is taking a step because it is being forced by the imminent trial date to make some financial payment, but that it is actually sorry and that it recognises that it has done the wrong thing and wants to attain."