A review by the Commerce Commission has found lenders are charging up to 803 per cent a year in interest rates.
The commission carried out research on the websites of 215 companies who lend money to the public last year to see whether they met certain requirements under the Credit Contracts and Consumer Finance Act (CCCF).
The CCCF Act did not cap interest rates, prescribe how lenders should name fees or impose a limit on the number of fees a lender may apply.
"We observed significant variation in interest rates and fees," said Commissioner Anna Rawlings.
"The review found interest rates ranging from no interest at all, all the way through to 803 per cent per annum. We also found more than 500 names used for fees ranging from $5 to $5,000."