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?"

Department of Transport land safety manager Brent Johnston said only a medical certificate stating a person cannot wear a seatbelt will stop those not wearing them being issued with a $150 fine.

"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.

Police agree.

"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 per cent, and by 44 per cent for those in the back seat, says Peter.

Source: AAP