New Zealanders might be paying too much for fuel, a government-commissioned inquiry has found.
The study showed profit margins for petrol at the pump have increased significantly over the past five years, while margins for aviation and commercial road users had been "flat or falling".
The report into the retail fuel market found "features which may not be consistent with a workably competitive market", Energy and Resources Minister Judith Collins said on Tuesday.
Ms Collins said the research is not confirming prices are unreasonable, but suggests there's reason to think that may be the case.
There were difficulties in comparing information received from the companies, she said, and some "very specific information that was required could not be obtained".
"As a result, the study doesn't definitively answer whether fuel prices are reasonable or not. However, the report does conclude that we cannot definitely say that fuel prices in New Zealand are reasonable, but we have reason to believe that they might not be."
The study showed that while the profit margin in fuel has risen, price differences are not explained.
It also found that higher petrol prices in the South Island and Wellington were not explained by higher costs in those areas.
It was a very complex area, the minister said.
"I have now instructed my officials to assess the recommendations of the study and report back to me by November."
Z Energy, BP, Mobil and Gull took part in the study.
Newshub
