It appears some drivers in Wellington and the South Island are paying more for petrol than those in the central and upper North Island.
In some cases the AA has said prices have ranged by as much as 40 cents a litre.
"And that's all the places where Gull and some of other cost operators have service stations, and in Wellington and the South Island companies like Gull don't operate there at all," says Mark Stockdale, AA Petrol Watch spokesperson.
The national price of fuel rose by five cents per litre this month and Government figures show the difference between what the fuel companies pay for petrol and what they sell it for has risen to 35 cents per litre.
The AA can't understand why.
"When we look at commodity prices and exchange rate movements we don't understand why prices rose by five cents," Mr Stockdale says.
"In fact, if anything the retail price should have fallen and not gone up."
Z Energy told Newshub its margins have not increased, saying it makes a profit of five cents per litre of petrol.
But the AA is calling on the Government to get involved.
The AA says there's been no independent Government review of petrol prices for more than eight years.
On Tuesday, new Energy and Resources Minister Judith Collins said she's disappointed fuel margins are going up and she'll be watching the issue very closely.