No clarity on how many owed back robo-debt

The department, now known as Services Australia, also doesn't know how much money it may need to repay.

More than 734,000 people have been overpaid welfare, according to income compliance reviews.

Ros Baxter from Services Australia said "a proportion" of these people would have been ensnared by the flawed income averaging technique of the robo-debt scheme.

The department has assigned 750 staff to manually determine how many people may be owed money.

"We are doing this as quickly as we can," Dr Baxter told a Senate committee in Canberra on Monday.

The controversial robo-debt system was recently declared unlawful by the Federal Court, which Dr Baxter said didn't impact the department's decision to scrap the scheme.

Dr Baxter couldn't say how many times Services Australia sought legal advice on robo-debt.

Earlier on Monday, the Australian Taxation Office said it collected more than $200 million in the past three years from the tax refunds of Centrelink recipients to recoup overpayments.

The office also used the process to claw back almost half-a-billion dollars in child care overpayments in that time.

Senior tax bureaucrats attributed an uptick in debt collections to an increase in notices being issued.

They assured the Senate committee the recovered cash was not just compromised of debts repaid through the robo-debt system.

When welfare debts are outstanding, the ATO receives a red flag from the department responsible.

"We don't go behind the debts - we're told that there's a debt," ATO executive Jeremy Hirschhorn told the committee.

"Whether it's lawful is relevant but we will inherently assume that a government agency issues lawful debts."

The tax office collected 40,000 Centrelink debts equalling $32 million in 2017-18.

This jumped to 74,000 debts the following year, raking in $63 million.

While Services Australia has frozen the robo-debt recovery scheme, the tax office will still collect debts from tax refunds.

Australian Associated Press