The Accident Compensation Corporation has admitted to overcharging about 9000 people for their car registrations.
Toyota, Nissan and Ford are among the eight makes of car that were rated and charged incorrectly for the risk of them crashing.
ACC chief customer officer Sid Miller said the issue was found last week within the coding that defines each car.
"We're urgently working through the fixes on this and I expect to have the fixes in place, certainly next week. We may have some in this week."
Mr Miller said they would be in touch with customers needing a refund over the next few weeks.
A full list of the vehicles affected can be found on the ACC website.