The number of infringement notices issued for illegally modified vehicles has increased by almost 50% in the past three years, police figures show.
The figures, obtained by Radio New Zealand, show 931 tickets in the 2007-08 year.
Last year, that number jumped to 1349.
Police national crash investigations adviser, Inspector Mark Stables, says the figures may not mean more modified cars on the road, merely that police are targeting them more.
The Low Volume Vehicle Technical Association, which oversees the certification process for modified cars, says there are increasing numbers of modified vehicles with warrants of fitness they should not have.
It says the warrant of fitness industry needs to be better informed of the requirement to refer modified vehicles for further inspection.