The price of diesel rose above 91 petrol this week for the second time in months, with experts saying a "weird combination" of global events is choking global supply.
They also warn it could get worse.
On Wednesday the average price of diesel rose to $2.57, nudging past octane 91 petrol priced at $2.56, according to fuel price tracking website Gaspy.
In August, the cost of diesel even surpassed the more expensive octane 95.
Gaspy co-founder Larry Green said the recent price jumps are unlike anything he's ever seen.
"To the best of our knowledge and certainly in the seven years that Gaspy has been in operation, diesel has never cost more than petrol," he said.
"In fact, until very recently it had not been even close."
While prices before 1986 were not immediately available, according to the New Zealand statistics website Figure.NZ, diesel has been our nation's cheapest fuel for more than three decades.
AA principal policy adviser Terry Collins said the soaring cost comes down to a "whole weird combination of things".
Global events, like the OPEC+ alliance of oil-exporting countries' decision to cut production earlier this month and a fuel refinery workers' strike in France, have raised the price of petroleum products including both diesel and petrol.
However, the reason why diesel now costs more than petrol can be traced to both the recent post-pandemic travel boom, and the arrival of the harvest season north of the equator.
"The northern hemisphere is using a lot of diesel right now because it's used in all the agricultural machinery and heavy transport trucks," Collins said.
"Post-Covid flights have taken off. Well, [jet fuel] is just kerosene."
He notes during the refining process, crude oil is broken down. Some of that extract, known as middle distillate, can be used to make either kerosene or diesel, but not both.
The end result - the recent demand for jet fuel has also squeezed the price of diesel.
But while those at the pump may be feeling the pain right now, he says the situation is only going to get worse.
"It's just the beginning. Next year's going to be really scary," he said.
"We've got to move to renewable energy. Currently there's still an increase in demand for hydrocarbons driven by populations moving to the middle class.
"And by end of the next year, particularly coming into the next northern winter, man, that could be really bad."
Infometrics principal economist Brad Olsen says the situation is "classic supply and demand".
"All of a sudden, your major providers are in the market and aren't able to provide quite as much so supply is down, demand hasn't changed, and that's forcing those prices higher," he explained.
However looking to the future, Olsen believes high fuel prices will be the least of our financial worries with the global financial market likely heading towards a recession.
"If interest rates keep rising at the pace they do, if the globe goes into a recessionary period, we're just not going to be demanding as much oil," he said.
"That will certainly limit how much higher prices go."
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