The Federal Court has ruled Australia's largest class action, against the ANZ bank, can proceed.

About 34,000 customers involved in the lawsuit had a partial win this morning when the court ruled some bank fees could be considered illegal penalty fees.

Justice Michelle Gordon ruled four fees charged by ANZ, including late payment fees, could be considered excessive and therefore illegal.

But she found several other fees, including overdrawn charges and dishonour fees, were valid.

Today's decision is an interim ruling and the case will return to court next year.

Perth-based litigation funder IMF took the $50 million class action against the ANZ, claiming its fees for overdrawn accounts and late credit card payments in the past six years were excessive and a breach of contract.

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One of the lawyers running the case, Andrew Watson from Maurice Blackburn, says today's ruling is a crucial decision for banking customers.

"This was one small step in the litigation, but I want to tell you it is one great leap for Australian consumers," he said.

"The fees we have won on are in relation to late payment on credit cards are significant fees for bank consumers everywhere.

"They are the bane, I think, of most of our existence."

Win

But ANZ chief executive Phil Chronican says overall today's judgment is a win for the bank.

"Thirteen out of the 17 fees - four out of the five fee types - were decided in our favour," he said.

Mr Watson contends the charges under litigation are excessive and unconscionable, but Mr Chronican says the bank does not believe that is the case.

"When a customer has paid late, that triggers the whole collections effort, so we have got people who call and follow up late payments plus there are additional things like additional credit charges that we have to apply, so that is something that will be heard in the courts next year when we get around to that," he said.

"We have consistently argued that although some of these fees may have been unpopular, they were lawful, and we are pleased that this has been vindicated in today's ruling."

Maurice Blackburn estimates the damages claimed in the class action could be as high as $50 million, and late credit card fees could account for about half of this figure.

But Mr Chronican says any forecasts are premature.

"It is hard to know exact numbers here because there is a whole lot of issues at stake," he said.

"It certainly won't be the full amount because there are real costs associated with this.

"So I think what we need to understand is this doesn't bring any closure to the court case at all.

"The costs debate has to be had next year, so it will be inappropriate for anyone to be estimating what amounts would be available to the customers."

Lowered

ANZ says it lowered the credit card penalty fee to $20 about two years ago to compete with other banks and be clear with customers.

Mr Chronican denies that is an admission the fee had been too high.

"We went through a look at all of our fees and looked at them against the competitors and we felt that in the context of lowering fees generally, $20 was the right amount," he said.

Mr Chronican admits ANZ was disappointed it was the only one of the big four banks to be taken to court on this issue.

"Because obviously that means we have incurred all the legal costs that are associated with that, but I guess from the point of view of the applicants they had to choose somebody and our name came first in the alphabet."

Maurice Blackburn says a decision on whether to launch fresh actions against other banks off the back of this case will be made shortly.