The ABC has apologised after admitting it has been underpaying some "flat-rate" casual staff.

Key points: A detailed review is underway to confirm how penalties, allowances and loadings should have been calculated

A detailed review is underway to confirm how penalties, allowances and loadings should have been calculated The ABC is notifying all affected employees

The ABC is notifying all affected employees The CPSU and MEAA unions are both involved

Head of public affairs Emma McDonald said in a statement that a detailed review was underway to confirm how penalties, allowances and loadings should have been calculated and applied in the cases of some 2,500 staff over the past six years

She said the broadcaster was notifying affected employees and reviewing its processes.

It is not clear how many of the 2,500 staff have been underpaid.

"The Fair Work Ombudsman has been notified and the ABC is liaising with them in relation to this situation," she said.

"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.

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

Sinddy Ealy, the ABC section secretary at the CPSU, said she expected the ABC to face a significant bill for underpayments, citing the case of one employee who the ABC admitted underpaying by $19,000.

"If there are 2,500 people affected and one individual with three years of employment under their belt was [underpaid by] $19,000, we're anticipating the liability to be sizeable," she said.

"It's disappointing, given the ABC has repeatedly responded to our concerns by assuring us they are paying casuals correctly.

"Our priority right now is to make sure that casual ABC workers are paid any back payments they are entitled to, and obviously to try to secure permanent jobs for some of these people, who have been working for the ABC for decades in some instances."

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.