The way the imported vehicle system is working is a "potential risk to safety", New Zealand Transport Agency (NZTA) says.
This is due to links between companies that both import and certify vehicles.
The agency said anyone with safety concerns about their vehicle should get checked by a certifier.
It is calling for industry submissions by the end of the month as it begins to crack down on conflicts of interest in the industry.
"A 'conflict of interest' means a situation where a person applying to us to carry out vehicle inspection or certification activities is or could be influenced or could reasonably be perceived to be at risk of being influenced, by professional, financial or personal interest in key parts of the vehicle entry and certification supply chain," it said in an online statement
Major testing company VINZ believes there is no perceived conflict of interest despite the fact it is owned by a Japanese company, Optimus, which also has importing interests.
NZTA is proposing that vehicle entry certification inspectors will not be allowed to check vehicles that someone linked to them has imported.
When the rules were created in 2014 "some integration of the used vehicle supply chain was anticipated and taken into account, but not to the extent that has subsequently occurred", it said.
"This means that one business could potentially be responsible for some or all parts of the vehicle entry certification process."
Its proposed new rules state it "recognises that the used vehicle entry certification process plays a critical role in promoting ... public safety by ensuring that only safe used vehicles are certified for use in New Zealand."
A whole new rule section would ban from importing and inspection any companies that have links such as the same ultimate holding company, common directors or shareholders, or even any "close personal relationship".
RNZ has identified three companies that import cars that also own compliance testing centres. However, the agency said that in each of these cases, the actual inspections at the site were being done by separate, approved inspection companies.