The Commerce Commission is investigating bank fees imposed on customers wanting to break their fixed-rate mortgage contracts.
Many customers are seeking ways out of their mortgage contract early in a bid to take advantage of falling lower rates.
It can cost tens of thousands of dollars to break a four-year fixed mortgage contract of $200,000.
Breaking a two-year fixed contract can cost more than $9,000.
The commission says it has received a number of complaints about such charges and has had many more forwarded to it by the Banking Ombudsman.
The commission has launched an investigation into bank fees under the Credit, Contracts and Consumer Finance Act.