Auckland solicitor Andrew Hooker said it had already had 6000 Kiwibank customers register since it launched the Fair Play on Fees Campaign in March this year and that number could double by the time it files the action.
Hooker said compared to its customer numbers Kiwibank had a high proportion of those who had registered to join the court action.
Comments received from Kiwibank customers also revealed a high level of unhappiness, he said.
"Some of the comments people were making...they were really annoyed."
Hooker said many had switched their accounts to Kiwibank to support the New Zealand-owned bank. "They thought that Kiwibank was going to save them from fees."
Hooker said there was a perception that Kiwibank was different and had been set up to take on the Australian-owned banks.
But instead it "seemed to be mimicking" the Australian-owned banks, he said.
Aucklander Leanne Briggs, who along with her husband Sydney, is the lead plaintiff in the case against Kiwibank said she had set up a joint account at Kiwibank six and a half years ago to "give them a go."
Over that time the couple were hit with 100 default fees totaling almost $2000. She quit the bank last year to return to ASB.
"It was just getting out of hand." Briggs who was charged a $10 default fee when her account went into overdraft by 2c after a mortgage payment came out, said it got so bad she couldn't tell her husband.
"I felt like I couldn't keep up with it."
Briggs said when she asked the bank if she could have an overdraft it told her that was only possible if she switched her mortgage to them.
A spokesman for Kiwibank said it had no comment to make on the legal case.
