The Commonwealth Bank has been accused of ripping off over 7,000 part-time workers.

The Finance Sector Union (FSU) says the bank's lowest-paid workers have short-changed by the compulsory employer component since 2009.

The FSU says part-time Commonwealth Bank (CBA) staff work branches, call centres and administration areas were being underpaid.

The union claims legal advice on the bank’s 9.5 per cent super policy will be used to launch a complaint with the Fair Work Commission.

According to the FSU, the part-time staff are being paid super for a set amount of hours each week, but do not get it got hours worked over and above the agreement.

A report late last year by Industry Super Australia and CBus found that about a third of Australian workers are not receiving their full superannuation entitlements.

The Commonwealth Bank said 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 spokesperson said.