The corporate regulator says a High Court judgement, on what is known as the Sportzone Motorcycles case, further clarifies what fees lenders are allowed to charge borrowers.
The judgement was a detailed, line-by-line ruling on which fees charged by Sportzone, and its financier, Motor Trade Finance, were reasonable and which were not.
Commerce Commission chairman Mark Berry said the ruling from Justice Toogood is the latest word on how credit fees may be calculated.
It follows an earlier ruling that some MTF fees had been unreasonable.
Dr Berry said the facts provide good case studies for lenders to consider.
Justice Toogood disallowed including the costs of training, travel, directors' fees, accounting, legal and audit costs in the fees charged to customers taking out loans to buy vehicles.
However, he did allow a number of costs, including 10 percent of the salaries of finance staff, some bank costs and some communications costs to be included in establishment fees.
Despite Justice Toogood's detailed assessment, MTF, which earns about $10 million a year from fees, said it still is not in a position to quantify any ultimate liability in respect of the Sportzone loans or the rest of its business.
MTF says that is because it is appealing both judgements.