The Commerce Commission has launched High Court proceedings against payday lender Ferratum which it says charged borrowers 183 percent in interest.
The commission hopes a judge will order some of the customers' loans to be reopened.
It said the company failed to help borrowers reach informed decisions about the loans, allegedly using pressure selling tactics and oppressive terms.
Ferratum failed to exercise reasonable care in its advertisements, it said.
Yesterday, the commission launched a review of the websites of 215 lenders to determine if they were complying with lender responsibilities.