National has been told to pull one of its online ads attacking Labour's Car Tax after the advertising watchdog ruled it was likely to "confuse or deceive" the public.
But the Opposition party told RNZ it "strongly disagrees" with the Advertising Standards Authority (ASA) and will appeal the decision.
The advertisement in question is one of a barrage of attack ads which National has rolled out on social media targeting the government's proposed "feebate" scheme and fuel efficiency standards.
The post - published on the party's Facebook page in July - accused the government of imposing a $6000 "car tax" and linked to a related press release from leader Simon Bridges.
The graphic was accompanied by text stating: "What hope does an ordinary Kiwi have for getting some relief from taxes and costs if the people running the government think $6000 is a small fee?"
It prompted two complaints from members of the public who argued the statement was "misleading or deceptive" as the policy stated the largest fee to be imposed was $3000 and only applied to a "very small number" of "the most gas guzzling" vehicles.
The ASA upheld the charges in late August and requested the post be removed, ruling that it was "likely to confuse or deceive consumers".
Responding to the complaints, National doubled-down on its claim and said the point of the ad was to "draw the viewers' attention to the [policy's] negative effect".
National said the linked press release put the $6000 figure in context and included the method and workings used to calculate the cost.
But the ASA complaints board said the proof provided by National was "insufficient" for the level of its "very specific claim".
It acknowledged the Facebook post qualified as "advocacy advertising" and so had slightly more leeway within the rules, but said absolute claims still required substantiation.
The board said its members had reviewed the government's consultation document and found "no evidence" that its plan could result in a $6000 cost.
The ASA also said it was not accurate to suggest the tax was directed towards "ordinary Kiwis".
A spokesperson for the National Party issued a short statement to RNZ, saying it objected to the authority's ruling: "We will comply with their request in the interim, but will be lodging a comprehensive appeal."
The decision follows another case last month where the ASA dismissed several complaints about another National Party ad criticising the "feebate" scheme.
That social media post compared a $1000 tax which would be slapped on a 1990s Corolla with a $1500 discount for an electric Porsche Cayenne.
In that case, the ASA ruled that while the ad used extreme examples, its point was ultimately substantiated.
The proposed "feebate" scheme, unveiled in July, would put an import fee on heavy-polluting gas guzzlers with the revenue used to subsidise clean, green vehicles.
The plan would run alongside new fuel efficiency standards, requiring importers to reduce the average emissions of the vehicles they bring in.