Z Energy has had no increase in its petrol margins over the past year and Prime Minister Jacinda Ardern's attack on petrol retailers' pricing is "inconsistent with Z's own experience," chief executive Mike Bennetts says.

He was speaking to BusinessDesk after Ardern claimed New Zealand motorists were being "fleeced" because of evidence she said showed petrol price margins had blown out at the same time as higher international oil prices and a new government regional fuel tax had raised prices to historic highs of around $2.50 per litre of 91 octane fuel.

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Bennetts cast doubt on Ardern's figures, which claimed a 9.8 cents per litre expansion in importer margins, based most probably on data routinely collected by the Ministry for Business, Innovation and Employment. The series is known for its volatility and tendency to be revised.


"If I go back to the potential fact sources at the heart of whatever claim is being made, I'd stress that it is provisional and it does get updated," said Bennetts. "The provisional data is inconsistent with Z's own experience, which will be made public when we announce our results for the half-year in the first week of November."

He declined to comment further, not wishing to give formal market guidance for the half-year result, which is due on November 1. Z Energy shares fell sharply after the Prime Minister's comments at her mid-afternoon post-Cabinet press conference, losing 2.4 percent of their value to close yesterday at $6.99, their lowest since late July.

However, Bennetts said Z Energy welcomed, and had always supported, a full Commerce Commission market study "when the law allows for that."

Ardern yesterday said the government would fast-track legislation in the next two sitting weeks of Parliament to give the competition watchdog powers to require companies to provide it with commercial information for studies into competitiveness in a market. She would promote a fuel industry review as the first such study.

Bennetts acknowledged the company's support for a study by a well-informed and experienced group of people, with access to all the necessary data and an understanding of the dynamics in the marketplace, would sound "a bit like blah blah" to motorists.

However, the investigation undertaken by the previous government and published in May 2017 had been hampered by constraints on time, access to data and "the experience of people of the dynamics of our market".

At the time of that investigation, undertaken by external economic consultants for MBIE, Z was known to have invested heavily in responding to information requests and its largest competitor, BP New Zealand had been compliant. However, Mobil was not forthcoming with information and Gull Petroleum claimed to be unable to provide data in the form the reviewers sought.

A Commerce Commission-led market study would be empowered to require companies in a sector both to provide data and to do so in a consistent form.

Even if started immediately after passage of the legislation, the outcome of a market study would not be known until well into next year, but Ardern may be hoping that by attacking petrol retailers, they will rein in their margins in response to growing political sensitivities.

The government has faced growing Opposition pressure to scrap recent new excises and a call for a one-day petrol station boycott.

- BusinessDesk