It is still unknown how many overpayment errors have been caused by Centrelink.

Figures provided by the Department of Human Services to the inquiry into Centrelink's automated debt recovery system show 133,282 debts were raised or identified between July and December last year.

So far, almost 8,500 of those have been reduced to zero or waived altogether.

Greens senator Rachel Siewert noted that the department had not collected any figures on how many overpayments came from errors within the system, rather than being caused by recipients.

“They don't count it,” she said.

“I asked again in estimates last week and they confirmed they don't keep a record of the number of errors that are made that result in overpayments, so we have no idea.”

The data-matching system that generated the fines was introduced in mid-2016.

It cross-references Centrelink information with Australian Tax Office records, and issues letters demanding an explanation if an apparent discrepancy comes up.

Human Services Minister Alan Tudge made changes  to address increasing community concern earlier this year, including sending letters by registered post to ensure delivery.

He also allowed people whose case is under review to avoid paying back the debt while the review is ongoing.

Senator Siewert said the changes did not go far enough.

“As far as many people are concerned it's like moving chairs on the Titanic for a better view, whereas what people want is a completely different system and they don't want the automated system, certainly not how it's being implemented even with the changes.”

The Australian Council of Social Services (ACOSS) says the program is causing “extensive distress and suffering”.

Mr Tudge denies the criticism that “a climate of fear” over debt repayments has emerged.

“These statements are being made by people like the unions and ACOSS who have a philosophical objection to doing widespread compliance checks,” Mr Tudge said.

“We think this is a critical part of our overall system to ensure there's integrity.”