Australia’s big four banks are being accused of gouging hundreds of millions of dollars in unfair fees from their New Zealand customers in a new class action.

The action, announced on Monday by law firm Slater & Gordon, will allege the largely Australian-controlled big banks in New Zealand have charged up to NZ$1 billion ($800 million) in excessive default fees.

It claims to be the largest class action in the country’s history, and its main targets are the NZ arms of the Commonwealth Bank, Westpac, ANZ and National Australia Bank.

Slater & Gordon lawyer Ben Hardwick said banks had charged their customers an average of $15 for overdrawing their accounts, paying their credit card bill late, or writing a cheque that bounced, whereas the real cost to the banks was ‘‘a few cents.’’

“These fees are clearly excessive and we estimate charges totalling up to $1 billion over the past six years. This case is about giving New Zealanders an opportunity to take a stand,” he said.

He said more than a million New Zealanders could be eligible to join the action – almost a quarter of the country’s population.

The class action is set to argue that various fees charged by the banks were unfair penalties that easily exceeded the cost to the bank of a customer overdrawing their account or paying late. Such fees can be deemed ‘‘unenforceable penalties,’’ the action will argue.

“We are not saying banks can’t charge fees, we are saying that they need to be fair and lawful,” Mr Hardwick said.

A separate campaign in Australia, led by law firm Maurice Blackburn, is alleging similar conduct by ANZ, Westpac, the Commonwealth Bank, NAB, St George, Citi, Bankwest and BankSA.

It has so far attracted a record 170,000 aggrieved customers.

The New Zealand action is targeting ANZ National, Westpac, Commonwealth Bank-owned ASB Bank, NAB-owned Bank of New Zealand and Kiwibank.

The original release of this article first appeared on the website of Hangzhou Night Net.