Jacinda Ardern 'conveniently' left out fuel tax figures - Simon Bridges

National Party leader Simon Bridges has claimed Prime Minister Jacinda Ardern doesn't know the cost of her own policies and "conveniently" left our fuel tax figures.

On Monday, in her weekly post-Cabinet press conference, the Prime Minister said New Zealanders were being "fleeced" by fuel companies, pointing to a 39 cent increase in petrol prices between in the last year.

The increase occurred between October 27, 2017 and September 28, 2018, and Ms Ardern said 6.8 cents of that could be attributed to taxes and levies, and the rest to importer costs and margins.

But, on Saturday, Mr Bridges said the Prime Minister had "conveniently" left regional and excise fuel taxes out of that figure.

"Staggeringly, this figure left out both the Government's excise tax increase of 4 cents per litre and the Auckland Regional Fuel Tax of 11.5 cents per litre," said Mr Bridges. "In fact, the 6.8 cents the Prime Minister referred to only includes GST and an ETS change".

Data from the Ministry of Business, Innovation and Employment (MBIE) shows the Emissions Trading Scheme (EST) taxes equals 1.77 cents, while GST is 5.04 cents, reports NZME.

MBIE said its methodology didn't take into account regional prices or regional fuel taxes, reports NZME, while the recent 3.5 excise tax didn't come into effect until September 30, two days after the data was collected.

A spokesperson for the Prime Minister said Ms Ardern's comments were accurate for the time period she specified and she mentioned the excise tax later in the press conference.

On Newshub Nation on Saturday, Energy Minister Megan Wood said only a "fraction" of the 39 cents was tax, but Mr Bridges says, for some New Zealanders, tax might be more than half of the increase.

"The actual increase in taxes is over 10 cents per litre outside Auckland, and more than 20 cents in Auckland, almost three times as much as the Prime Minister claimed".

Mr Bridges said Ms Ardern was using the fuel companies as a scapegoat for high petrol prices, of which taxes were a "key driver".

"How can the Prime Minister claim to understand the impact her policies are having on New Zealanders when she doesn't have her numbers right," he said.

Concerns about anti-competitive behaviour in fuel market mean the Government will prioritise passing the Commerce Amendment Bill when Parliament resumes next week.

The Bill will compel companies to produce information to the Commerce Commission to fully understand how markets are functioning.

The study will report back in 2019 and the Government will prioritise a response to it.