SYDNEY, Aug 12 (Reuters) - An Australian law firm is widening a consumer class action case, already the country's largest, to target five major local and international banks over credit card fees worth as much as A$250 million ($231 million).

Maurice Blackburn said on Tuesday it had filed papers in the New South Wales Supreme Court against Australia and New Zealand Banking Group Ltd, Westpac Banking Corp, Westpac units St George and BankSA, and Citigroup Inc's Citibank over the late payment fees it claims were excessive and unfair.

The law firm said it expects to file further suits against Commonwealth Bank of Australia, CBA subsidiary Bankwest, National Australia Bank Ltd and American Express.

The wider action follows a test case late last year involving 38,000 ANZ clients in the Federal Court in Melbourne.

Lawyers for ANZ's customers argued that the bank's exception fees, including late payment, overdrawn and missed payment fees, were penalties and were unfair and unconscionable.

The Melbourne court ruled in favour of the ANZ customers, a decision that is being appealed by the bank.

The expanded action is open to any customers of the targeted banks who have ever paid a late payment fee.

"We're talking about an enormous action," Maurice Blackburn lawyer Andrew 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."

(1 US dollar = 1.0811 Australian dollar)

(Reporting by Jane Wardell; Editing by Kenneth Maxwell)