Fuel vouchers and loyalty schemes may be helping turbo drive petrol companies' profit margins and working against the interests of consumers, a government report says.

Labour economic development spokesman Stuart Nash said people would be better off overall if they were done away with completely.

A report produced for the Ministry of Business, Innovation and Employment found gross profit margins on fuel at the pump had doubled in Wellington and the South Island over four years.

The study's authors argued discount vouchers offered by the likes of supermarkets and loyalty schemes could be part of the problem, as they allowed petrol companies to "price discriminate".

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"In imperfectly competitive industries, charging different types of customers different prices for the same product can increase profits.

"However, it does not improve outcomes for consumers," the report said.

The effect of vouchers and loyalty schemes was to push up the regular "board" price of fuel, as advertised on petrol stations' sign boards, while reducing some of the forces of competition, it said.

The report was written for the ministry by economists and consultants at NZIER, Grant Thornton and Cognitus Economic Insight.

Petrol companies had been increasing their use of discount and loyalty schemes, they said.

"On the data available to us it has not been possible to evaluate whether the increasing use of price discrimination is net positive or net negative for consumers."

Nash said consumers would be better off overall if such schemes were scrapped, as it would increase the pressure on petrol companies to offer "everyday low prices".

An aggravating factor was that some discount schemes could involve "the poor subsidising the wealthy", he said.

"At my local supermarket they have a deal once a month if you spend over $200, you get 10c or 20c off. There are a whole lot of families that simply cannot afford to spend $200 a week on groceries."

The debate has put the AA in the seemingly somewhat unusual position of siding with petrol companies over pricing tactics.

The AA runs its own petrol discounting scheme in conjunction with BP and Caltex.

Spokesman Mark Stockdale argued the "popularity" of such schemes showed they were in motorists' best interests.

"In some of those schemes the discount is not all funded by the fuel companies," he said.

But he agreed more research might be useful.

"If it were possible to identify to what extent price discounting and loyalty discounting was contributing to margins that would be an interesting exercise – but we simply can't answer that," he said.