The program has identified $2 billion worth of debts to the Commonwealth, Services Australia has told a parliamentary inquiry.

Since it began in 2015, more than one million reviews have been completed. Of those, about 734,000 had a debt.

"[This] means that four out of five people involved in a review did not accurately tell us about their past income and personal circumstances," the department said in its submission.

So far, $785 million of the outstanding debts had been recovered, and a further $725 million was under a repayment arrangement. The average debt was $3000 in 2018-19.

The department was chasing another $500 million, mostly from "non-current customers".

"Protecting integrity in this way has cost approximately $606 million," the department said.

The admission follows evidence to Senate Estimates in February indicating the cost of the scheme nearly matched the money it had been able to recover.

It also comes after a class action against the government for the debt collection system was announced.

But the department was keen to point out most of the cost of the process reflected the "human involvement", unlike the image the nickname "Robodebt" projected.

In 2018-19, about 84 per cent of the costs of running the scheme related to salary and administration.

The department also made changes to its policy in the last year, which meant reviews of more than 100,000 "income discrepancies" that would have resulted in low or no debts were finalised without contacting the client.

"This was possible through the use of the outcomes of previous reviews, specific data points and modelling the likely outcome. These reviews would previously have been initiated and the customer engaged," the department said.

Meanwhile the Commonwealth Ombudsman revealed it received more than 10,000 complaints about Centrelink last financial year.

While the Ombudsman said complaints about Robodebt had fallen significantly since improvements to the system made in 2017, it still receive about 100 a month in the year to June.

The Community and Public Sector Union said there had been a rise in aggression at Centrelinks since the government began garnisheeing tax returns without telling clients first.

"Some customers are getting their entire tax return garnished from old debts and this has already caused customer aggression in my office and we are a low level aggression site," one staff member told the union.

"Unsurprisingly, the increased workloads and customer aggression associated with rRobodebt is affecting the morale of staff, many of whom sympathise with customers and disagree with the raising of inaccurate debts. There is no job satisfaction for staff, morale is at an all-time low. It has taken a toll on DHS staff, because we can see that Robodebt was a flawed strategy from the very beginning."