Centrelink debt cases at the federal appeals tribunal have soared by more than 50 per cent since mid-2016 and The Greens have laid the blame for the surge, which might take years to work its way through the system, squarely at the feet of robo-debt.

But the giant Department of Human Services, which runs Centrelink says that there is nothing out of the ordinary going on and its "Online Compliance" system is attracting appeals at a lower rate than the usual manual process.

Centrelink has been embroiled in controversy for more than six month over its online debt-raising system which automatically compares information clients supply to the welfare agency with data supplied to the Tax Office and raises debts based on an any perceived discrepancies.

The effort began in earnest in mid-2016 and by January 2017, more than 230,000 letters had been sent out.

A top official from the Administrative Appeals Tribunal told Senate Estimates last week that 4354 challenges to alleged Centrelink debts had been taken to the Administrative Appeals Tribunal between July 2016 and April 2017.

The total for the whole previous year for debt disputes was just 3387 out of a total of 13,198 applications to the tribunal for reviews of Centrelink decisions.

Policy costings prepared by Treasury for the online compliance policy anticipated the AAT would be left with a bill of more than $3.3 million for its share of administering robo-debt, but that the cash-strapped tribunal would have to find the money itself.

The AAT's executive director of Strategy and Policy Christopher Matthies told Senate Estimates in Canberra that it was a "significant increase" but said the tribunal's case management system could not tell the difference between robo-debt appeals and challenges to debts raised manually.

But Greens' Senator Rachel Siewert has little doubt that robo-debt is to blame.

"This is clear evidence that the Centrelink's debt recovery system is resulting in more problems and errors such that members of the community are having to appeal", Australian Greens Senator Rachel Siewert said.

"I suspect that many of these are resulting from the robo-debt debacle.

"There is so much evidence suggesting that the robo-debt system is fundamentally broken in its rollout and is causing so many problems.

"The government shouldn't be compounding stress for people who have accessed the social safety net for support as a student, a parent, a carer, a job-seeker or a person with disability."

In response to questions, a Human Services spokesperson said the number of appeals this financial year were broadly in line with the usual volume.

"The total number of applications to the Administrative Appeals Tribunal (AAT) (First Review) are broadly in line with previous years," she said.

"In the 2016–17 financial year to 31 March 2017, approximately 98 per cent of appeals to the AAT were not as a result of online compliance.

"The number of AAT requests for Online Compliance Interventions compared to the manual process in the previous financial year is much lower, at 0.1 per cent compared to 0.7 per cent."

Figures from the AAT show 13,894 appeals lodged against Centrelink decisions in the first 11 months of 2016-2017 against a total of 13,198 in 2015-2016, an increase in the monthly average of about 15 per cent.