The ABC on Thursday conceded making an error calculating penalties, allowances and loadings for flat-rate casual staff.

A Community and Public Sector Union complaint prompted the ABC to refer the matter to the Fair Work Ombudsman.

"This error should not have occurred and the ABC apologises to any casual employee who has been underpaid," it said in a statement.

"The ABC is actively working to remedy this for affected employees as soon as possible."

CPSU ABC section secretary Sinddy Ealy said the scale of the mistake over so many years would be unacceptable in any workplace, let alone the national broadcaster.

"Clearly the total bill for underpaid wages is going to be substantial and comes at a time when the ABC can ill afford it," Ms Ealy said.

Last month, the ABC admitted underpaying a Brisbane-based employee from its news division by $19,000 over three years.

The ABC assured the union in 2016 casual employees were being paid properly.

Ms Ealy said those assurances, which came in response to concerns about the broadcaster's over-reliance on casual workers, meant people responsible for the underpayment needed to be held accountable.

"Clearly the ABC dismissed the CPSU's concerns without even checking if there was a problem," she said.

The CPSU fears some workers will not get backpay, with compensation claims limited to the past six years despite widespread use of casual workers for two decades.

"All workers affected by this disgraceful situation should be provided with backpay, and many of these workers should also be given permanent jobs," Ms Ealy said.

Australian Council of Trade Unions president Michele O'Neil slammed the ABC, calling for meaningful consequences for "wage theft".

"This is truly appalling conduct from the ABC, the underpayment of huge amounts of money from thousands of employees is outrageous and will never be tolerated," Ms O'Neil said.

The ABC has agreed to work with the CPSU and the Media, Entertainment and Arts Alliance to address their concerns.

Australian Associated Press