The big difference between the price of petrol in cities compared with the regions is one of the issues Energy Minister Judith Collins wants oil companies to explain.
Ms Collins on Thursday announced an investigation into the price of petrol and the return against cost - the difference between what the companies are paying for it and what they are charging for it at the pump.
"One concern I have is why people in Levin pay around 20c less a litre than people in Wellington," she told RNZ on Friday.
"I want a bit of an explanation of these regional and city differences."
Ms Collins decided to hold the investigation after the Ministry of Business, Innovation and Employment told her its figures showed a 100 per cent increase in fuel margins in the last five years.
"The oil companies don't agree with some of the MBIE figures," she said.
"I want to make sure there's no price fixing or rorting, or super profits, and that people throughout the country are paying a fair price."
Ms Collins said the four big oil companies had told her they'll cooperate fully.
"They are international companies, they know what they're doing, they should be able to provide the information."
One of the big four, Z Energy, responded to Ms Collins announcement by welcoming the "clarity" the investigation would provide.
Chief executive Mike Bennetts says MBIE's figures presented a misleading picture and the company's profit per litre had not increased over the past year.