The Morrison government's "robo-debt" bill could run to hundreds of millions of dollars, as it faces calls to provide welfare recipients with more information about how it will review old debt recovery decisions which relied on invalid methods.

On Wednesday, in a landmark court case, the federal government admitted a key element of the beleaguered robo-debt system is unlawful. The admission follows last week's news that the Morrison government would no longer rely solely on averaged income data from the tax office to raise debts, and would review hundreds of thousands of robo-debt cases.

Government Services Minister Stuart Robert said the government will work through old robo-debt decisions "sensibly". Credit:Alex Ellinghausen

Government Services Minister Stuart Robert has previously said only a "small cohort" of people will be affected by the reviews, but Services Australia data provided to the Senate suggests at least 220,000 cases stand to be re-examined.

On Thursday, in the wake of the federal court development, Mr Robert did not respond to questions about how many people the government will now need to contact and what the time frame is for doing so.