The High Court is about to begin hearing Australia's largest class action case with 38,000 customers taking on the ANZ bank over its fees.

Lawyers for the customers are expected to tell the court today that penalty fees charged by the bank are out of proportion with the actual cost.

The action covers various fees, including those for overdrafts, overdrawn accounts, dishonour fees and over-the-limit credit card accounts.

Penalties or fees?

The court will be asked to determine whether the charges are penalties or fees for a service.