Illegal car traders operating at roadside venues and car fairs are slipping through the Government's net, according to the Motor Trade Association.
In the past month six people were convicted of illegal vehicle trading, but MTA warns the figure could be much higher.
MTA spokesman Tony Everett said while the increase in prosecutions was welcome, it still only represented a small portion of those who operated outside of the law.
"Each year, roughly 450,000 cars change hands between private sellers. Given the size of the market, it is likely the number of illegal traders operating is considerably larger than MBIE's figures indicate."
Many illegal traders operating at roadside sales venues and car fairs were slipping past the attention of government agencies, he said.
Under the Motor Vehicles Sales Act 2003, individuals can legally import only three, or sell only six, vehicles in a 12-month period without being registered as a Motor Vehicle Trader.
Since July 2012, The Ministry of Business, Innovation and Employment has investigated 358 cases of possible illegal vehicle trading.
Of these, 279 have been finalised, with 61 still under investigation and a further nine before the courts.
This meant only 2.5 per cent of all cases investigated ended in prosecution, Mr Everett said.