Associate Transport Minister Julie Anne Genter ordered an investigation into a total ban on petrol car imports by 2035 among a range of options to reduce emissions in the national transport fleet.
An array of documents, including an excerpt from a draft cabinet paper from April last year, reveal Genter instructed officials to work on the policy, before scrapping the plan.
But National says the fact the plan was even worked on was "irresponsible".
A Ministry of Transport (MoT) report from September last year showed officials had been instructed to "progress an initiative to regulate an end-date of 2035 to the import of light vehicles that are unable to be driven without fossil-fuels".
In other words, Genter wanted to ban the import of all cars which still use petrol by 2035.
This was an idea first floated by the independent Productivity Commission, Genter said.
"It's normal for officials to investigate and recommend a wide range of options to ministers," she told the Herald in a statement.
MoT officials progressed work and presented a report in October last year titled "Ban on Importing Light Vehicles Powered by Fossil Fuels; Preliminary Cost-Benefit Analysis".
This report looked into the pros and cons of such a ban.
But, due to the "lack of information, time and resources", the cost-benefit analysis was not able to take a number of key issues into consideration.
For example, how such a ban would affect EVs was not looked into, nor was the impacts on electricity prices and the road safety impacts of changing New Zealand's vehicle fleet so dramatically.
The report said the main benefit for consumers would be the saving people would make when it comes to not having to buy petrol.
"The Government is prioritising making it easier and more affordable to choose electric and hybrid cars, while ensuring Kiwis can also choose from a wide range of efficient petrol and diesel options," she said.
Last month, Cabinet gave the green light to a feebate scheme whereby the Government would slash the price of imported electric and hybrid vehicles by up to $8000 in a bid to make greener cars cheaper for Kiwis.
But it is also planned to slap a new fee of up to $3000 on the import of vehicles with the highest greenhouse gas emissions.
National transport spokesman Chris Bishop said the fact a cabinet paper was produced showed she was working behind closed doors to change the law.
"Reducing emissions from our vehicle fleet is an important step in the fight against climate change.
"But it would be irresponsible to make petrol cars illegal so soon without a solid plan to help people into electric vehicles," he said.