Being too overweight to fit a standard seatbelt isn't an excuse for not wearing one, police and transport authorities say.
Seatbelt extenders, like those widely used by overweight plane passengers, are available for cars and could save lives, they say.
The issue came to the fore after an Auckland woman was filmed refusing to wear a seatbelt because it didn't fit across her waist.
She argued seatbelt laws discriminated against overweight people.
"I can't fit it, so what' the point of wearing it. If they're going to give me a fine for not fitting my seatbelt, that's just being racist, to fat people," the woman said in a video posted on Facebook by her sister Aalysia Tupou.
"What do they think the whole world is skinny or something? What about the big people?"
Land Transport safety manager Brent Johnston says you can only avoid a $150 fine if you have a medical certificate stating you cannot wear a seatbelt.
"We recommend people seek alternative options if they are unable to use a standard issue seat belt, for example fitting an extender belt," he says.
"Wearing a seatbelt is not optional," road policing operations manager, Inspector Peter McKennie says.
Of the 327 people killed on New Zealand's roads last year, 90 weren't wearing a seatbelt, up from a previous average of 60.
Wearing a seatbelt reduces the risk of death or serious injury of front seat passengers by 60 percent, and by 44 percent for those in the back seat, according to Inspector McKinnie.