Kiwi motorists are being warned against getting caught with illegal vehicle light modifications - with those selling them often neglecting to mention they can't actually be used on the road.

Car modifications sold online in New Zealand may not be legal for the purposes they are suggesting. Source: 1 NEWS graphic

Head and tail light modifications are common in New Zealand's car scene, and are often done to give the components improved aesthetics or performance.

Modifications include High Intensity Discharge (HID) headlight conversion kits, which replace standard bulbs with units capable of boosting light up to 300 per cent, "Angel Eye" headlight rings, LED bulbs, and head or tail light tint films.

The items are available variously from online retailers and car modification shops, including Jaycar Electronics, TradeMe and many others.

Of the numerous online advertisements for modifications seen by 1 NEWS, many had wording suggesting they could be used on New Zealand roads without problems.

"HID provides far greater light output than standard automotive lights," a product listing on the JayCar Electronics website reads, "at night, road signs, pedestrians, and general road hazards will receive much better illumination so you can drive with more confidence."

NZTA spokesperson Andy Knackstead warned that "in most cases" these items are in fact illegal to use on New Zealand roads.

"There is a risk that modified or illegal lights could dazzle or confuse other road users, and there is also a risk that tinted lights may not be adequate for a driver to see the road properly, or be seen by other drivers," Mr Knackstead said.

Motorists caught using illegal modifications on the road could receive a fine of $500 if convicted in court, or a $150 infringement notice on the spot.

"These products can be sold 'not for road use', and should be advertised/marked as such," Mr Knackstead said.

Headlights are required to produce "the same light as at manufacture," he said, which rules out most modifications.

Darkening tail lights too much using "shadow" or "smoke" tint film or sprays could lead to a failed inspection, as brake lights need to be visible from a distance of 100m in daylight, and 200m at night.

The only tint film legal on tail lights is red, so long as the brightness doesn't go below the aforementioned levels - there is no legal requirement for reversing lamps to be white or for rear indicators to be orange, but any colour other than red would likely fail inspection.

A spokesperson for the Ministry of Business, Innovation and Employment said: "If traders are representing the products as able to be fitted on to a car, then this may be a false or misleading representation under the Fair Trading Act".

JayCar Electronics spokesperson Dean Williams said their HID products are actually targeted at show cars and off road vehicles, but said they have added additional warnings to their website after being contacted by 1 NEWS.

A TradeMe spokesperson said the site agrees some listings in these categories are "a little ambiguous" and urged buyers to do their due diligence before buying.

"We're going to take a look today and, if necessary, speak to sellers about their obligations and remove any listings if we think that's necessary," they said.

In addition to possible fines, NZTA's Vehicle Inspection Requirements Manual says vehicles should fail a WOF if "a headlamp is fitted with a type of light source other than that specified by the vehicle manufacturer or the headlamp manufacturer".