The Senate has passed a motion calling on the government to table legal advice that presumably warned it has unlawfully demanded repayment of hundreds of thousands of alleged debts over several years, based on an obvious flaw in its robodebt process leading to inaccurate calculations.

The Community and Public Sector Union says the government has been ignoring the concerns of Centrelink staff about the issue for years.

The Greens tabled the motion on Wednesday, after a landmark Federal Court decision showed the government must have recently decided that income averaging, a process it relied on for years to generate hundreds of thousands of alleged debts, was legally unjustified.

The motion tabled by Senator Rachel Siewert notes the Senate does not accept legal professional privilege as an acceptable reason to refuse, and asked for the information to be tabled by 10am on Wednesday. That did not happen.

The government claims it would be against the public interest to release the advice and argues this is in line with long-standing practice, an excuse that Siewert rejects.

“Clearly this Government has something to hide,” she said on Thursday. “They continued with this draconian program long after the harrowing evidence of the Senate inquiry in 2017 and the recommendation that it be put on hold while issues of procedural fairness were dealt with and long after the major flaws with the income averaging process were identified.”

“We need to know when and for how long the Government had information on the legality of robodebt.”

On Wednesday Siewert said the Federal Court decision raised questions about the legal status of the robodebt program to which the parliament and the community deserved answers. “Many of us suspect there is no basis for the government to issue these debt notices,” she added. “The Government need to front up and take responsibility for what they have done.”

This dramatic change of legal position was also apparent in last week’s sudden decision that Centrelink would stop claiming people owed very specific amounts of money, which were actually only very rough estimates based on averaging their annual income out evenly across a particular financial year.

With entitlements to income support being based on periodic reporting of actual income earned, the agency’s staff were well aware that in the absence of more detailed information, from payslips for example, this would produce inflated or completely false debts.

The government took the position that if the person could not or would not provide that more detailed information, it would proceed to demand payment of the inflated or totally false debt.

The CPSU says it is aware of over 600,000 individual debts that are now “in question” and says frontline Centrelink staff were warning their bosses that “serious problems” would arise from the ramped-up debt collection program before it began.

National secretary Melissa Donnelly says those workers will struggle to “provide clarity and comfort” to people who call up asking what happens now.

“There will be hundreds of thousands of Australians rightly confused this afternoon, and the Morrison Government must provide clarity to the community and staff about exactly what this order means to all other debts, and what the government is going to do about it,” she said on Wednesday.

“This decision will put more pressure on an already understaffed workforce. CPSU members are calling on the staffing cap to be lifted so that they can clean up the mess the Morrison government has created.”

Read more: Robodebt defeated: Commonwealth caves in and accepts debt and 10% penalty both invalid