More people are not wearing seatbelts and they are being killed in car crashes, warn police.

Assistant Commissioner of Road Policing Dave Cliff said police are extremely concerned by the very high numbers of people - especially women - being killed in crashes because they aren't wearing seatbelts.

The number of people killed on New Zealand roads while not wearing restraints has almost doubled in the past two years, Cliff said.

"That is an unacceptable statistic and for that reason all police districts will be putting extra focus on ensuring seatbelts are worn this year.

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"Almost a third of people who die on our roads are unrestrained and these are all deaths which are entirely preventable.

"We are at a loss to understand why there has been such a massive increase in people not wearing their seatbelts."

Last year, 96.5 drivers and adult front-seat passengers wore seatbelts.

This was a drop from the 97.1 per cent observed in 2014 and is the first time there has been a reduction in the rate since 1996.

Also of concern to police is the substantial increase in the number of women dying in crashes while not wearing a seatbelt in the past five years.

"If people don't listen up and get the message in 2017, we will likely see more than 100 people lose their lives and thousands more be injured completely needlessly," Cliff said.

"All this can be prevented by taking two seconds to put on your seatbelt, it's that easy."

Cliff said not a road in New Zealand was immune from crashes.

"Anyone who chooses not to wear a seatbelt only increases their risk of injury or death if they end up in one."

A seatbelt reduces the risk of death risk by 50 per cent in the front seat and 75 per cent for back-seat passengers.

"It's a no-brainer. So let's all be sure to 'buckle up buttercup' every time you get into a vehicle.

"Make sure your friends and family do the same so we can avoid more preventable deaths on our roads."