ANZ and ASB may have to refund interest to more than 100-thousand customers with home loans.
Plaintiffs are seeking the refund after the banks breached consumer rights contracts.
The Court of Appeal has ordered affected customers will be automatically included in the case.
Class action lawyer Fionnghuala Cuncannon told Heather du Plessis Allan the banks failed to comply with their disclosure obligations.
ANZ hasn't opposed the decision, but says it has already rectified the situation with customers, paying out more than 35 million dollars.
ASB says it does not comment on litigation.
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