Waka Kotahi has updated its safety ratings for most car models based on recent real-world accident data from across New Zealand and Australia.
The ratings judge the likelihood of injuries in a crash.
The agency's principal safety advisor Todd Wylie said a lot of people did not realise how unsafe their vehicles were.
"It's a star rating system, with five stars being the best and one star being the worst," he said.
"Around 40 percent of our fleet are one- and two-star rated vehicles, and we know that those vehicles are highly over-represented in crashes.
"We know that occupants of five-star vehicles are more than twice as safe than they are in one star vehicle."
The overall safety rating a vehicle received took into account how safe a vehicle was for both its occupants and for other road users, Wylie said.
"Some vehicles obviously look after their occupants really well, but they can actually be really aggressive against other vehicles that they come into contact with."
It was a "common misconception" that safe vehicles cost more, Wylie said.
"Safe vehicles are available in most price ranges."
But he said new vehicles entering the fleet were "generally safer".
"As well as providing better protection for their occupants in a crash, they also have an increased amount of crash avoidance features."
However the age of a vehicle did not necessarily determine its safety rating.
"There are safer cars that are older, and we just really encourage people to check the safety rating of their own vehicle."
Wylie said drivers could check the safety rating of their vehicle - or any vehicle they were interested in purchasing - on the government's Rightcar website.