The Commission said BNZ "likely breached responsible lending and disclosure obligations under the Credit Contracts and Consumer Finance Act 2003.
It comes after BNZ reported 15 potential breaches of the act to the Commission.
The breaches relate to home loans, personal loans, credit cards, and overdrafts spanning June 2015 to February 2017. A total of 11,956 customers were affected.
Commission chair Anna Rawlings said BNZ "provided incomplete or inaccurate disclosure, and in other cases disclosure was provided a day, a few days, or as many as seven months after the information should have been provided to borrowers".
Rawlings added: "We expect lenders to regularly audit their systems to make sure that they can comply with consumer credit law, or quickly identify problems if they arise, fix them and provide appropriate remediation to borrowers."
In late 2018, BNZ refunded approximately $3.8 million in interest and fees to borrowers affected by these issues, the Commission said.
The Commission said BNZ had cooperated throughout the investigation.
In September this year, the bank refunded $350 to each borrower affected by the failures. The refunds, to 2,300 borrowers, amounted to $1 million.