Centrelink’s “robo-debt” system is a form of illegal extortion allowed by failings across a “plethora” of democratic and legal institutions, according to a former member of the administrative appeals tribunal.

Prof Terry Carney, a long-serving member of the AAT, has penned an extraordinary attack on the institutional failings that allowed the robo-debt program.

It’s the second time Carney, who helped oversee the writing of Australia’s social security laws, has used academic journals to condemn the system as illegal this year.

Carney’s last paper said robo-debt involved the enforcement of “illegal” debts that in some cases were inflated or nonexistent, an allegation that was forcefully rejected by the Department of Human Services. Hank Jongen, the department’s spokesman, said at the time that the department “strongly refutes any claims that it has conducted its compliance activities in a manner which is inconsistent with the legislation”.

This time, Carney used a piece in the Alternative Law Journal to map out the numerous shortcomings that allowed the system to come into being and operate for 18 months without challenge.

“The pivot for this article is not so much that Centrelink lacks legal authority for raising virtually all debts based on a robo-debt ‘reverse onus’ methodology rather than use its own information gathering powers – for this remains essentially uncontested,” he wrote. “Rather it is extraordinary that this went unpublicised and uncorrected for over two years.”

Centrelink has long used a system of automated data-matching to detect discrepancies in income reported by welfare recipients, to detect and claw back overpayments. But it introduced significant changes from July 2016, reducing human oversight and expanding the system considerably in a bid to recover more debts and improve the budget. The new system effectively shifted the onus onto the welfare recipient to prove they owed no debt to the government.

The system spat out letters to individual welfare recipients as soon a discrepancy was detected in their reported income to Centrelink and records held by other agencies, like the tax office.

A flawed process was used to calculate their debt if they did not respond or could not produce evidence of their previous pay, which involved averaging out their yearly income across all 26 of Centrelink’s fortnightly reporting periods. The process often led to the false assumption that a welfare recipient had worked across an entire year and was ineligible for social security, thereby creating a debt.

Carney argues the rushed design of what he described as a “machine-learning budget ‘savings measure’” trumped good design standards. He says inquiries by the auditor general and the commonwealth ombudsman into the system had failed to consider whether it was raising debts on a lawful basis.

Carney also argues that Centrelink, by pursuing debts raised through the controversial “income averaging” technique, has failed to adhere to ethical administration. He says Centrelink has continued to use this method, despite knowing AAT rulings that it is invalid.

“It breaches principles of ethical administration regarding avoidance of oppression of vulnerable and uninformed citizens,” he wrote. “To continue to do so constitutes moral bankruptcy and surely warrants the label of acting in an ‘extortionate’ fashion.”

The privacy safeguards in the first tier of the AAT mean that most legal challenges against welfare debts are not publicised, he writes. That means that “rulings overturning Centrelink reasoning remain hidden from the public”.

Carney says advocacy bodies and legal aid have been starved of resources and undermined, weakening their ability to challenge the system, and that pro bono legal services and civil society groups have failed to fight the government on the illegality of the system.

He also argues the lack of a sympathetic attitude to the vulnerable means the illegality of the system “has largely failed to catch public attention”.

Jongen, in response to Carney’s latest piece, said Centrelink’s debt recovery system was reasonable, lawful and fair. He said there were protections in place for welfare recipients, and they had appeal and review rights to protest a decision.

“They include safeguards, which seek to ensure the final decision made is the correct one,” he said.

“If a person gives us information that shows no debt or lesser debt is owed, we’ll use that to inform our final decision. If a person is dissatisfied with a debt decision, they have formal rights of review they can pursue at any time.”

Jongen said a review of the system by the Commonwealth Ombudsman found it was reasonable and appropriate for welfare recipients to be asked to explain discrepancies in their reported income.

“Debt recovery is a fundamental principle of our welfare system – when someone is overpaid, whether through fraud or not reporting a change of circumstances – they must repay that money,” he said. “This is money that belongs to hardworking Australian taxpayers and is needed for things like funding new life-saving treatments through the Pharmaceutical Benefits Scheme.”