The Automobile Association is warning Kiwi motorists to expect high fuel prices for at least another six weeks, and possibly through to Christmas.

The price of 91 unleaded petrol on Friday hit an average of $3.25 a litre on fuel monitoring site Gaspy.

That's an increase of nearly 25 cents a litre over the last 28 days and is the highest average petrol price since late May.

Global crude oil prices have climbed steadily over $US100 ($NZ175) a barrel in recent weeks as oil supplies from key Middle East producers continue to be constrained by the ongoing conflict between the US and Iran.

AA principal policy advisor Terry Collins said a raft of factors were now combining to push up the landed price of petrol in New Zealand.

He said this included restricted supplies through the Strait of Hormuz, attacks on Saudi oil pipelines, soaring insurance costs for oil tankers and a lower Kiwi dollar.

He said that meant there was likely to be a little more pain to come.

"We probably won't get much relief before Christmas, unless something really major happens geopolitically," Collins said.

Collins said one big factor, however, in terms of what oil prices did before Christmas, would be the US midterm elections on November 3.

Should US President Donald Trump's Republican Party lose control of the US House of Representatives and the Senate at the election, it could weaken the power of the White House.

Collins believes Iran knows this and is doing all it can to restrict global oil supplies now because it wants to keep prices high for motorists in the US, in the hope that it would damage the Republican Party's chances.

"They've got no military might that they can attack the USA. But what they can do is make them pay through the ballot box by forcing up the price of diesel. It's at record prices in the US. It's never been this high per gallon," Collins said.

Diesel in New Zealand is currently sitting at $2.96 a litre, still well short of the peak earlier this year of around $3.80 in April.

Collins did not believe New Zealand would see a return to those sorts of levels, as much of that price spike was driven by the initial uncertainty and volatility around the war.

However, he said there was a lot of global competition for diesel at the moment, and local prices were likely to rise further.

"For diesel, and that's where globally everybody's competing, those prices are going to keep going up. That runs our trucks, our harvesting equipment and the rest of the world. There are also flow-on effects around fertiliser and a whole bunch of other impacts."

When it comes to getting supplies of petrol into the country, Collins is confident New Zealand is not likely to face any major issues.

He said the current issue is a price problem, not a supply one.

"What we've done is the government and the industries have worked really effectively in putting in place a plan around storage. That includes additional diesel and increased tanks up in Channel Infrastructure up in Marsden Point. It means we have really accurate data on our shipping, and the ships coming in, and we are managing our load," Collins said.