The Government's plan to make greener cars cheaper could also end up subsidising vehicles with lower safety ratings, the National Party says.
But Associate Transport Minister Julie Anne Genter has accused the Opposition of "desperately scaremongering".
The Government last week proposed a policy that would see lower-emitting vehicles, and especially electric cars, discounted by up to $8000, while "gas guzzlers" would be hit with price increases of up to $3000 from 2021.
National leader Simon Bridges says many of the smaller lower-emitting Japanese vehicles that could get subsidies under the scheme had one or two-star safety ratings.
The Government is reported to be mulling a ban on lower-rated cars.
The Vehicle Standards and Certification Reference Group, one of several advising the Government on an upcoming safety plan, has listed the restriction as a top priority.
"These cars will be cheaper under Julie Anne Genter's feebate scheme," Bridges said.
"She doesn't seem to realise many of the vehicles she wants to subsidise with her car tax may be banned from entering the country because of their low safety rating."
Bridges points to the Suzuki Swift, the pre-2014 version of which would be banned under the safety proposal, but get an $1100 discount under the clean cars plan.
Suzuki has defended the safety of its car, saying non of the new Swifts coming into the country are rated below four stars.
Higher-emitting but safer vehicles, such as Toyota Camrys and Ford Rangers, meanwhile, would be made more expensive, Bridges said.
Genter said it was "simply untrue" to say unsafe cars would be subsidised.
"All imported cars have to meet minimum safety standards," she said.
"Different models of the same car have different safety ratings - many versions of the Suzuki Swift for example, have 5-star ratings."
The Government has yet to announce its new road safety strategy.