It says when it implemented changes under the CCCFA in 2015, some confirmation letters weren't sent quickly enough and some documentation was missing information - such as total interest and total payments. Some new credit card and home loan accounts were set up with old documentation because the rules changed while the paperwork was being processed.
Chief executive Angela Mentis said then that no customers had been overcharged "but we didn't get it 100 percent right."
BNZ now says it voluntarily made the regulator aware of these issues "and we've been proactively engaging with them on these matters. However, as it's an ongoing investigation, we're not able to comment further at this time."
Neither could BNZ say whether it has raised the matter with the Financial Markets Authority and Reserve Bank joint inquiry into banks because material provided "is subject to a confidentiality order."
But BNZ's initial engagement with the Commerce Commission about CCCFA compliance took place before the FMA/RBNZ inquiry began last year, it says.
The Commerce Commission closed a similar investigation into Westpac in October after a settlement including agreed changes to certain consumer credit contracts.