One of Australia's largest banks faces a multimillion dollar payout to thousands of customers after a judge ruled on Wednesday that late payment fees it charged on credit cards were exorbitant.

ANZ Banking Group Ltd. partially lost a class action law suit in the Australian Federal Court brought by more than 43,000 customers who claimed they had been charged excessive fees for years. In some cases the fees were 70 times the cost to the bank of administering late payments.

Justice Michelle Gordon ruled that the bank had been illegally imposing penalties for late payments on credit cards.

She agreed with lead plaintiff Lucio Paciocco's argument that the fees were "extravagant, exorbitant and unconscionable," and represented a breach of contract.

But she also ruled in ANZ's favor by dismissing claims that other types of bank fees were illegal penalties.

It was not clear how much the bank would have to pay back customers who had been charged too much over six years. Lawyers for the bank and customers have until next week to agree on a proposal for repaying customers that the court can rule on.