Six rogue car dealers have been banned from trading for five years for serious breaches, including failing to comply with orders from the Motor Vehicle Disputes Tribunal.
In the past year, the Ministry of Business Innovation and Employment prosecuted and fined 18 traders for unregistered trading, including seven in the last month alone.
The Registrar of Motor Vehicle Traders and manager of Trading Standards, Stephen O'Brien, said the number of "cowboy" traders was increasing, largely due to the growing online motor vehicle market.
"The purchase of a motor vehicle is likely to be one of the largest purchases a consumer will make and it is vital that consumers have confidence in the industry," Mr O'Brien said.
"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."
Under the Motor Vehicle Sales Act 2003, registered motor vehicle traders are required to display a Consumer Information Notice, keep a record of the contract for sale, and prohibit tampering with the odometers of a motor vehicle.
Mr O'Brien said the ministry's main objective was to get "voluntary compliance" from traders.
"In the cases where the trader does not engage with the registrar, or refuses to comply, we will investigate and take the necessary action."
Consumers can check if a trader is registered online through Trading Standards Motor Trader website.