In an email to staff on Thursday,chief people officer Rebekah Donaldson apologised and promised the ABC would remedy any confirmed under-payments as soon as possible.
"We recently identified that some casual employees have been underpaid,"Ms Donaldson said.
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"A detailed review is underway to confirm how penalties,allowances and loadings should have been calculated and applied over the past six years to about 2500'flat-rate'casual staff.
"Current and former casual employees who might be affected are being notified and the ABC is reviewing its processes to address the issue for the future."
The Australian Council of Trade Unions accused the ABC of"systematic wage theft ... from an organisation that should be a best-practice employer".
The broadcaster's admission comes as it battles a budget black hole that saw it record a loss of $71.2 million in 2017-18 - significantly worse than the $28.7 million loss it originally forecast,largely due to redundancy payouts.