An "enormous" class action will be lodged against a number of major banks and institutions over unfair credit card late fees.
Compensation law firm Maurice Blackburn will lodge the action on Tuesday on behalf of hundreds of thousands of bank customers.
The action against ANZ, Citibank and Westpac will be filed in the NSW Supreme Court, with more against other banks and institutions to follow.
Maurice Blackburn principal Andrew Watson says the open class action will include all customers of ANZ, Westpac and Citibank who have ever paid a payment fee.
The action is different to a closed class case the law firm lodged against ANZ earlier this year, which was only on behalf of people who had signed up to the action.
The action is likely to be worth hundreds of millions of dollars.
"We're talking about an enormous action," Mr Watson said.
"If people are a bit like myself and not as careful about paying off their credit card, then they will be in the action and stand to benefit."
Mr Watson says more actions are to come.
"In the near future we'll be filing further claims against other banks and they'll be on the same basis," he said.
Maurice Blackburn successfully obtained a judgment, which is now subject to appeal, against the ANZ earlier this year, Mr Watson said.
"But we are confident that we'll obtain compensation," he said.
CEO of the Consumer Action Law Centre Gerard Brody told ABC Radio on Tuesday morning that it could be Australia's biggest class action against banks.
He said the case builds on the federal court finding in March that late fees charged by ANZ were unlawful.
"This case should bring some justice to consumers," he said.
He said while banks charge varying fees for late payments in Australia, some charge up to $20 or more.
But the federal court in the ANZ case found the actual cost of administering the late payment was "more like 35 cents".
"These sort of late payment fees can't be all out of proportion," he said.
"They could charge a $1 or $2 fee, and that would still probably be lawful.
"But we shouldn't have late fees worth 20, 30 or 40 dollars."
As the fees are often hidden, Mr Brody said customers might be getting an unfair deal, particularly as interest on credit cards continues to be charged.