The Senate's community affairs committee is holding another inquiry into the system, which is facing continued calls to be scrapped.

National Social Security Rights Network solicitor Kathleen Boyle told senators about a case where a man born with an intellectual disability was required to prove he did not owe Centrelink $14,500.

The man - whose mother gave permission for Ms Boyle to share his story - works as a supermarket trolley collector and was asked to provide pay slips dating back to 2012.

"If he didn't have his mother or us, he would be paying back that debt without it being properly established if he owed that money," she told senators.

His case is currently being appealed.

Centrelink knows he has a disability because he receives welfare for that reason, Ms Boyle said.

"It is exploitative of people with disability, because they really have no understanding of what's going on," she said, pointing to a lack of transparency in Centrelink's calculations.

Senior government official Jason McNamara confirmed people on disability pensions were targeted for possible welfare debts, as the system doesn't leave certain types of payments out.

He said the current system included improvements from its first version, based on feedback from people using the software.

For instance, the system now allows people to save information as they go.

Centrelink had the power to contact banks and employers in order to get pay information about welfare recipients, Mr McNamara added.

But he couldn't say how many times the department had contacted a bank on someone's behalf, promising to provide senators with the information at a later date.

Centrelink only contacts people if they might owe more than $1000, he said.

The latest inquiry is the second in less than three years, while the scheme is also facing a class-action lawsuit.

Robo-debt is expected to cost more than $500 million over the next three years, for a benefit of $2 billion to the federal budget.

The scheme matches tax office and Centrelink data to claw back overpaid welfare payments.

Ms Boyle told senators about a case where a family received a robo-debt for their relative who had passed away.

Labor senator Deborah O'Neill said 80 per cent of debt notices were repaid, questioning if people were too scared to challenge the government over its figures.

Liberal senator Matt O'Sullivan questioned why people had trouble accessing bank statements from institutions they no longer had accounts with.

"Remember, we're not talking about people who are very confident or able to advocate for themselves," Ms Boyle said.

Ms Boyle recalled one instance where someone had to provide pay slips dating back 10 years, while other people were told they had to pay back welfare when a debt collector turned up at their house.

Department officials insisted their staff worked directly with people who needed help finding past pay information.

The government has admitted about one-fifth of debt notices included information that was later proved to be wrong.

Australian Associated Press