The Banking Ombudsman is urging people to check the fine print before they sign up for any contracts, including a new fixed rate mortgage.
It is receiving complaints from people about break fees and early repayment charges.
"We are seeing a lot of early repayment cases where customers could have saved themselves a lot of heartache and money by understanding what they are signing up for," Banking Ombudsman Nicola Sladden says.
Lower interest rates are an incentive for people to break an existing mortgage.
But when a customer pays off a fixed rate mortgage early it can cost the bank money. That is because when interest rates are falling the bank cannot re-lend those funds at the same rate that it had originally loaned the money.
So the banks are allowed to charge fees to cover costs from a mortgage being repaid early.
However this must be set out in the terms and conditions.
The bank won't be able to tell you how much the charges will be when you take out the loan. The potential fees will depend on interest rates at the time of the early repayment of the loan.
The bank has to say in the contract that an Early Repayment Charge might be imposed and how it will be calculated.
The fee must also be "reasonable."
The Commerce Commission looked at this issue in 2010 and judged that in general the fees were reasonable.
The Banking Ombudsman says that in the last financial year lending related complaints accounted for a third of the cases it dealt with.
Ten percent (103 out of 1,006) of the lending-related cases it received in the 2015-16 financial year were about early repayment charges. Most of those (61 percent) were received in the first four months of the financial year (July to October).
Its website features 30 quick guides covering common banking complaints.