Organisers of a campaign targeting what they call unfair bank fees say 37,000 people have signed up.
Fair Play on Fees is planning court action to recoup money charged as default fees.
Campaign group lawyer Andrew Hooker said Kiwibank account holders have until 11pm on Thursday to sign up. Papers are lodged with the High Court on Friday.
ANZ customers are also part of the legal action, and have until 13 December to join.
Mr Hooker said proceedings against other banks are pending.