Banking customers are losing out, according to a draft report that finds a lack of competition.
The Commerce Commission's found ANZ, ASB, BNZ and Westpac also have sustained high profits compared with global peers.
The report says Kiwibank also lacks the capital to disrupt the big four in a meaningful way.
Commerce Commission chair John Small says these banks aren't interested in competing with each other.
"They're not really going after each other, they're matching each other's prices, they've got really high visibility into what each other's doing- they're typically aiming to maintain market share, rather than take market share off each other."
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