Over 24,000 New Zealanders have registered to take part in the lawsuit, of whom 11,000 are clients of either ANZ or the National Bank business, which was formally merged into the ANZ last year.
Hooker said yesterday he estimated up to $250 million could be clawed back from the ANZ based on fee calculations in Australia.
"ANZ has been overcharging its customers millions of dollars over the last six years and we want to provide an avenue for people to get that back."
Hooker said he would file court papers next Tuesday against the ANZ and encouraged other customers of the bank who wanted to take part to sign up to the legal action by 11pm on Monday, June 24.
But the ANZ, which last year reported a record underlying profit of $1.37 billion for the year to September 30, said it would defend the claims.
"We will be vigorously defending the claim. We're very upfront about our exception fees. They are set out in our terms and conditions, which are provided to our customers," Thompson said.
She said most people did not pay penalty fees.
"It's a very small proportion."
She encouraged the 11,000 people who had signed up for the case and any others who were unhappy about fees to talk to the bank.
ANZ New Zealand's parent company, along with 11 other banks, is facing a similar lawsuit in Australia which has yet to be resolved.
Thompson said New Zealand was a different jurisdiction with different fees.
ANZ is not the only bank expected to be targeted.
Hooker yesterday confirmed the other major banks ASB, BNZ and Westpac as well as state-owned Kiwibank were also in his sights.