Prime Minister Jacinda Ardern's calculation of how much extra tax Kiwis are paying at the petrol pump on Monday did not include the recent excise tax or Auckland's Regional Fuel tax.
National Leader Simon Bridges said the Prime Minister has got this "badly wrong," and has made a "staggering mistake."
But a spokesman for the Prime Minister said her comments were "based on the most accurate information Ministry of Business, Innovation and Employment (MBIE) had compiled at that time."
At a press conference on Monday, Ardern said consumers were being "fleeced" at the petrol pump, blaming this on the increased margins of petrol companies.
Between October 27, 2017 and September 28 this year, petrol prices have risen 39c, according to MBIE data – Ardern said just 6.8c of that increase was due to "taxes and levies."
That 6.8c increase is made up of a 1.77c increase in Emissions Trading Scheme (EST) taxes and 5.04c of GST over the same period, MBIE data shows.
But the 10c a litre Auckland Regional Fuel Tax and 3.5c a litre fuel excise tax, introduced on September 30, were not included in the "taxes and levies" side of Ardern's equation.
In a statement to the Herald, an MBIE's spokeswoman said: "current methodology does not accommodate regional prices or regional fuel taxes" in their fuel price calculations.
"Auckland City has recently introduced a regional fuel tax that will increase fuel prices in the Auckland region. Our current methodology does not accommodate regional prices or regional fuel taxes. We are developing a new methodology to replace our existing methodology that will include regional retail price differences in its measure."
As for the 3.5c excise tax – that came into effect on September 30 and the MBIE data Ardern was referring to was taken from between September21-28, meaning it was not included in MBIE's data either.
"Excise tax is included in our fuel price data, but obviously only once it has been introduced," an MBIE spokeswoman said.
In a statement, a spokesman for the Prime Minister said Ardern based her comments on information supplied by MBIE.
"She was clear that the figures she was using were accurate as at that time and were through to 28 September 2018."
"The most up to date data was through to 28 September and the excise increase did not come into force until 1 October. The Prime Minister elsewhere in her answers at post-cab referenced that 3.5c increase."
Bridges said in trying to defend her new fuel taxes, Ardern has shown "she doesn't even know how much they are costing New Zealanders."
"The Prime Minister has been trying to blame fuel companies but a key driver of petrol prices is her Government's higher taxes."
On Monday, Ardern pointed the finger at fuel companies and their high margins as one of the main reasons for a 39c increase in fuel prices between October 2017 and September this year.
"As a moral stance, I think New Zealanders are paying too much for petrol," she said.
But BP's New Zealand Managing Director Debi Boffa pointed the finger right back at the Government.
"Recent price changes have been influenced by increases to the cost of product and the weakening NZ dollar. In addition to this, the government recently introduced the Auckland Regional Fuel Tax plus increased the national fuel excise by 3.5 cents per litre (plus GST) impacting the price of fuel for motorists in Auckland and across the country."