No 'reasonable explanation' why petrol prices are so high - Jacinda Ardern

Prime Minister Jacinda Ardern says she hasn't been given a "reasonable explanation" why petrol costs so much now.

Prices have gone up 40c a litre in the past 12 months, and the Opposition has urged the Government to rescind its latest fuel taxes - including a 3c excise tax increase and Auckland Council's 10c regional fuel tax, which required a change in legislation to implement.

"We've seen a real spike in recent times. It's something personally I have been worried about and keeping a close eye on," Ms Ardern told Newshub Nation.

She has asked the Commerce Commission to look into what can be done to bring prices down, but is adamant the fuel tax will stay.

"The jump that they have seen cannot be answered by that regional fuel tax. I can't guarantee that if we removed excise or the regional fuel tax, that we wouldn't see that price at the pump continue to stay high.

"One thing I can guarantee though is that every cent from the regional fuel tax and excise - every single cent - goes straight back into roading, straight back into the kind of public transport projects that will give commuters options, that will stop them sitting in traffic, that will stop that lost productivity."

Ten years ago, New Zealand had one of the lowest pre-tax fuel costs in the OECD - we now have the second-highest.

Oil prices are much lower now than they were in 2011/12, when prices at the pump last peaked at around $2.10 - they're now around $2.45.

"Things have happened in the last 10 years and no one's been able to provide me a reasonable explanation," said Ms Ardern.

"Excise and the regional fuel tax doesn't actually answer for us why we've seen a 40c increase in the last 12 months. Neither does transportation costs or the change in the currency or the price of Brent crude."

Ms Ardern says the extra cost put on lower-income consumers by the tax increases is more than offset by changes the Government has made to Working for Families and the minimum wage.