The Commerce Commission says its ongoing consumer credit dispute with Motor Trade Finance is its most significant case for establishing rules on what fees lenders can charge.
Mark Berry, chairman of the anti-trust regulator, told the commission's annual conference this week that consumer credit remains a priority area.
It will use the courts to clarify the boundaries after recent changes to the Credit Contracts and Consumer Finance Act 2003, which came into full effect this month.
The commission is in a legal battle with the car finance business and Sportzone Motorcycles over alleged breaches of the CCCF Act and the Fair Trading Act.
"The consumer area has been one where there has been a particular uplift in work, following legislative amendments in recent times," Berry told the Competition Matters 2015 conference in Wellington.
"We are looking to be active in terms of taking cases to establish relevant precedents in this area.
"The most significant one is the MTF case which is litigation still in process."
The commission alleges that the finance businesses breached consumer protection law over fees charged in 39 loan contracts originated by Sportzone between May 2005 and July 2008.
Sportzone had an agreement with MTF allowing the motorcycle business to write credit contracts for buyers of motorcycles.
"This case is important because it sets the rules relating to the limits on what can be charged for various fees under this legislation and we have a Court of Appeal ruling which is subject to appeal in the Supreme Court," Berry said.
"We see that we do have an important role in bringing test cases to get clarity around principles and legislation that doesn't tell us all that we need to know."
In September and October the High Court issued two judgments in favour of the commission that some fees in the lender's loan contracts were unreasonable, while rejecting other aspects of the commission's claims that MTF and Sportzone, which is in liquidation, failed to make proper disclosure of components of credit fees payable under the loan contracts.
The Court of Appeal upheld the High Court judgments and dismissed MTF and Sportzone's appeal after a hearing in November.
However, this month the Supreme Court granted MTF leave to appeal the Court of Appeal's decision.