Four people operating unregistered vehicle sales businesses have been prosecuted in the past month by the Ministry of Business, Innovation and Employment (MBIE).
The ministry said the cases were a reminder it was taking action in this area.
"Unregistered motor vehicle traders are not subject to the checks that apply to those who are registered and consumers may have less protection when something goes wrong," Karla Flood, manager of the registry integrity and enforcement team at MBIE, said.
Compliance was the first objective, but the recent prosecutions were a reminder to the vehicle trade that action would be taken against those who did not abide by the rules.
The recent run of prosecutions started with a 52-year-old man being convicted and fined $5400 in North Shore District Court in early March for selling 10 vehicles without being registered as a motor vehicle trader.
Six days later, a 21-year-old man was found guilty in the Auckland District Court of the same offence, this time for selling 12 vehicles, and fined the same amount.
A 32-year-old man found guilty at Gisborne District Court for selling 13 vehicles received 80 hours of community work, and early this month a 24-year-old man was convicted and fined $8000 at Auckland District Court for selling 10 vehicles.
MBIE said the number of unregistered vehicle traders was increasing largely due to a change in trader behaviour towards online markets.
The Registrar of Motor Vehicle Traders at MBIE worked with Trade Me, Customs, and Land Transport New Zealand, to help identify unregistered traders. Since last August, 17 unregistered vehicle traders had been convicted.
The number of new registered vehicle traders had increased by 379 during the same period.
Since July 2012, the registrar had investigated 422 unregistered motor vehicle traders. Of those investigations, 339 had been finalised, 74 were ongoing with the aim of achieving compliance, and nine cases were before the courts.