Official figures claim the margin on petrol increased again in 2016, prompting calls for an investigation.

Energy Minister Judith Collins has vowed to "get to the bottom" of rising petrol margins, signalling an announcement of an inquiry could come next week.

As official figures suggest the margins Kiwis are paying to fill up at the pump rose again in 2016, and have doubled over the past decade, the AA claims two price rises in January came despite the price petrol companies pay being flat.

The AA has called for petrol companies to explain what is going on, with areas including Wellington and much of the South Island frequently charging motorists 30c a litre more than towns such as Levin and Masterton, despite being further from where fuel is imported.

ROBERT KITCHIN/FAIRFAX NZ Energy and Resources Minister Judith Collins has vowed to "get to the bottom of" rising petrol margins.

Collins, who became Energy Minister at the end of 2016, said she had immediately asked officials at the Ministry of Business, Innovation and Employment (MBIE) about the issue of rising margins.

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On Tuesday Collins met with officials to discuss petrol margins and an announcement of "actions" could come next week.

"I will be taking some suggestions through to my colleagues, but I think there will be some actions announced next week," Collins said.

"I am like any other person in New Zealand concerned if these margins continue to rise when there seems little justification for it.

"I will get to the bottom of this and next week I expect to have something to announce."

As well as generally rising margins, Kiwi motorists are increasingly paying vastly different amounts depending on where they live.

While the so-called 'national price' - which is typically faced in Wellington and the South Island - is around $2.09 a litre, the upper North Island is generally cheaper, with reports motorists in Rotorua were paying as little as $1.60 a litre last week.

AA spokesman Mark Stockdale said petrol companies needed to explain a 5c a litre rise in the so-called "national price" of petrol in January, taking the price to $2.09, which he said was without justification.

"None of those increases had anything to do with a rise in the commodity price [of petrol] or the exchange rate," Stockdale said.

"It's completely disconnected from any change in the imported cost of fuel."

​Stockdale said there appeared to have been a sharp increase in regional discounting, typically in areas near where Australian fuel discounter Gull has petrol stations.

Motorists in Wellington could be being charged to make up for a lack of profitability elsewhere.

"Other parts of the country are helping fund those discounts."

The figures published by MBIE claim the weekly average importer margin on regular petrol rose from about 28 cents a litre at the start of 2016, to 30c by the end of the year.

The margins represent the difference between the cost of importing fuel and the average price motorists pay. It does not include the cost of delivering fuel to the petrol stations, or any other costs of operating a fuel company.

While MBIE revised how it measured the petrol margins in 2015 after protests from the industry, the fuel companies continue to dispute the figures, claiming they do not fully account for the amount of discounts which are offered to motorists.

"The level of off [price] board discounting...is really increasing," Z Energy spokesman Jonathan Hill said. Z Energy also supplies the Caltex petrol stations, making it by far the largest fuel supplier in New Zealand.

The figures which MBIE publishes weekly "paints a misleading picture about what margins are at this point in time," Hill said.

While he agreed the fuel industry was now making more profit than it was seven years ago "when the industry was effectively bankrupt", Z Energy's margins in 2016 were "absolutely flat" compared to 2015, Hill said.

The company would cooperate fully in any inquiry if it were to take place.

"We would participate fully in it and would in some respects would welcome it."

British-owned BP would not make any of its staff available for interview, but said in a statement that there was "significant" discounting in some parts of the country.

"[W]e will independently change our prices when and where we can to ensure competitiveness in the markets in which we operate," spokeswoman Shelley Brady said.

"Prices can differ all around the country on different days and at different times, but we cannot always undertake or sustain heavy localised discounting."

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