The Commonwealth Bank has been accused of refusing to pay the full employer superannuation entitlement to more than 7,000 part-time workers.

The Finance Sector Union (FSU) claims some of the bank's lowest-paid workers have been "ripped off" on the compulsory employer component since 2009.

FSU national secretary Julia Angrisano said the underpaid part-time Commonwealth Bank (CBA) staff work in branches, call centres and administration areas with set hours each week.

"The CBA has been ripping off some of its lowest-paid staff since at least 2009," Ms Angrisano said.

"We are committed to make sure the bank pays up.

"It's bad enough that Australia's most profitable bank pays some of the lowest wages in the banking sector — now we are finding that part-time staff aren't getting their full superannuation entitlement."

The union has received legal advice on the 9.5 per cent super underpayment and plans to lodge the complaint with the Fair Work Commission unless the superannuation is paid back.

According to the FSU, the part-time staff impacted have been paid employer super to cover a set amount of hours but have been denied superannuation for hours worked over and above the agreement.

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Majority of underpaid workers women

Ms Angrisano said 89 per cent of the underpaid Commonwealth Bank workers were women who did additional hours to manage staff shortages and peak periods.

"They are the lowest-paid, they are the frontline staff and we believe they have contributed to the bank's profit-making machine," she said.

"They deserve their superannuation to be paid — we're determined to get justice for our members."

The latest allegations embroiling the Commonwealth Bank follow a report late last year by Industry Super Australia and CBus showing around a third of Australian workers are being ripped off on their superannuation entitlements.

The Commonwealth Bank said the complaints were being examined but that the 7,000 figure cited by the FSU was inflated.

"We have assured the FSU that CBA understands the importance of superannuation to help secure and enhance the financial wellbeing of our employees, and are committed to ensuring that we comply with obligations to employees on this issue," a spokeswoman said.

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