Key Points:

The Commerce Commission has begun investigating banks over the fees they charge for late payments on credit card bills.

A spokeswoman said levies were being checked against the Credit Contracts and Consumer Finance Act, which says late payment fees must be "reasonable".

Payments have soared from about $10 seven years ago to to $49 today, on top of nearly 20 per cent interest rates for most credit cards. Most banks appear to charge about $25 a month for missed minimum payments.