Motorists are finding ways to get around petrol prices - with the biggest discount card now saving buyers cardholders to $85 million a year.
Petrol price margins have more than doubled since 2004 to around 30 cents a litre.
The government is considering launching an inquiry into growing petrol price margins.
The new Energy Minister Judith Collins has met with officials about this, and said she was disappointed the margin was growing.
She said people had a right to know what they were paying for when they fill up their car at the petrol station, and it is important to get the facts clarified.
"Because there is some dispute between some of the companies and MBIE about the margins, so it's really important to get that clarified.
"And people have commented and have noted that petrol prices seem to vary from station to station quite dramatically for no discenable reason."
Ian Sutcliffe who lives in Auckland is finding ways to cut his petrol bill via a discount card.
He paid nothing for a tank of gas last weekend and his AA Smartfuel card is already back up to a 58-cent-a-litre discount.
"I was filling up with super so... it was $2.08 a litre I got off, so I got $104 off. We pretty much get a free tank of gas every five weeks."
Typically, cardholders claim 6 cents a litre off if they spend $40 at the pump, buy petrol in in $40 lots, join cards in family groups and use them to buy groceries too.
One reason Ian Sutcliffe is so good at juggling his fuel discount card, backed up as it is by cards his wife and son have, is that he's a director of the company Smartfuel that invented the cards.
AA Smartfuel managing director Scott Fitchett started things off at his Palmerston North petrol station in 2008 when customers began disappearing due to supermarket dockets.
He looked to Australia and the United States for ideas. In the five years since its nationwide launch the card has doubled the total savings being delivered on fuel to a million cardholders to $85 million a year.
Some say it is difficult to juggle such a card to get the best discounts - and life would be easier if petrol were just cheaper all-round.
One of those is Alex Buchanan, a taxi shift supervisor from Wellington. Fuel prices in the capital are high - 32 cents a litre more than in Levin and 46 cents higher than Rotorua - the sort of gap that has prompted claims some parts of the country are being subsidised by others.
"My last fill was I think $1.14 per litre off. I filled 50 litres and saved about $54.
"The problem with that is jumping through all sorts of hoops to have to get there... going on special days, filling to the minimum amount as many times as you can until those points are expiring, and then cashing it in with a minimum fill.
"Be much better just to get a fair price."
Marquerite Williams of Masterton finds using supermarket dockets less confusing than a discount card. Until recently she carpooled every weekday to her graphic design job in Wellington and filled up twice a week.
"You get very savvy about who has what specials. So Z will have double Fly Buys, then I get a triple whammy. So I pay with a voucher, get 35 cents off a litre, swipe the Fly Buys, and then you get Air Points when you pay with your credit card, and then you can convert those again for petrol vouchers too."
Masterton has cheaper fuel than Wellington as there is more competition there from minor players. Ms Williams thinks Gull is cheapest at the pump, but by the time she factors in all her discounting, Z is the best bet - and her AA Smartfuel card comes third because it works only at BP or Caltex which often had higher pump prices than Gull.
Other price-discounting innovations are up to 15 cents a litre off for Supergold card holders at South Island pumps run by NPD, which also pays out up to 12 cents a litre on any supermarket docket.
The real mover though is AA Smartfuel. The 50-50 joint venture between Scott Fitchett's company and the AA has 2.4 million cardholders, with about a million of those regular users of it.
"Probably we would be on a worldwide scale the biggest on a per capita basis of any type of programme around fuel."
But the New Zealand Institute of Economic Research has repeatedly found no basis for another big criticism on fuel prices - that retailers pass on oil price rises at the pump much faster than they do price drops. In fact, it's about the same.