The ABC has apologized and is contacting about 2,500 "flat-rate" casual staff the network underpaid over the past six years.

Officials say a detailed review was underway to confirm how penalties, allowances and loadings should have been calculated and applied.  Workers with both the CPSU and the MEAA were involved, and one worker was underpaid $19,000.

"The Fair Work Ombudsman has been notified and the ABC is liaising with them in relation to this situation," said Head of public affairs Emma McDonald.  "The Corporation is having discussions with the CPSU, which brought the matter to the attention of management, and intends to work with affected staff, the CPSU and the MEAA to address their concerns.

"This error should not have occurred, and the ABC apologises to any casual employee who has been underpaid," she continued, "The ABC is actively working to remedy this for affected employees as soon as possible."

Last year the Federal Government announced it would freeze the ABC's annual funding indexation for three years from July 2019, costing the organisation $84 million.