The cost of importing petrol and the price New Zealanders pay for it at the pump has the largest discrepency of all developed countries in the world.
As it stands, New Zealanders are paying 21 cents a litre for the petrol over what it cost fuel companies to import it - even after tax.
This margin is the largest of all OECD countries.
In Wellington, and the South Island the price margin is even larger, paying up to 30 cents a litre over the petrol import cost.
A Ministry of Business, Employment and Innovation study found there may not be a "workably competitive market" for the sale of petrol in New Zealand.
Retail margins have increased "significantly" in the last five years – but those in the South Island and Wellington have increased at a faster rate than margins in the rest of the North Island.
"This could mean fuel prices in the North Island are subsidised by prices across the rest of the country," the study says.
Gross margins for fuel not sold to the public, for example in aviation or to commercial road users like trucking, have been flat or are declining.
But the report doesn't definitively conclude prices are unreasonable – because MBIE says not all the data was comparable across companies or provided in time for the report.
Energy Minister Judith Collins has asked officials to examine the report – and she says the Commerce Commission could take a look.
"We cannot definitely say that fuel prices in New Zealand are reasonable, but we have reason to believe that they might not be," Ms Collins said in a statement.