For decades, diesel has been noticeably cheaper than petrol at the pump.
But that longstanding price dynamic has been flipped on its head by the conflict in Iran, which has disrupted global fuel markets and sent prices skyrocketing.
On average, 91 octane petrol is costing $3.42 a litre, while diesel has edged ahead at $3.43, according to user generated fuel price tracking app Gaspy.
In the past 28 days, petrol prices have risen by 35.77% – while diesel has surged by 85.09%, more than doubling petrol’s rate of increase.
Diesel, a thicker and denser fuel, is more expensive to produce than petrol because of the way crude oil is refined. From every three barrels of crude oil, refiners typically produce about two barrels of petrol and only one barrel of diesel.
That made diesel inherently costlier, but taxes on petrol have historically masked that difference for New Zealand motorists, AA principal policy adviser Terry Collins told 1News.
"We don't see that over here, because we put fuel excise duty and ACC and some other regional taxes on petrol. Traditionally in the past, those taxes have made the price of petrol more expensive."
Nicola Willis outlines phase one of fuel security plan as stocks remain at normal levels. (Source: 1News)
In January, diesel refining costs made up around 14% of the final price in New Zealand. Collins said that figure had now jumped to more than 40%, a rise of more than 200%.
Petrol refining costs have also increased, but much more modestly – by about 60% over the same period – allowing diesel prices to surge ahead.
"Although we tend to look at oil as the big driver, of late, it’s actually been the refining cost."
Diesel was also far harder to substitute than petrol.
Globally, it is relied on for freight transport, farming, construction and food production, with few short‑term alternatives available. When prices increased, demand tended to remain strong because businesses could not easily switch fuels or reduce usage.
New Zealand’s love of diesel‑powered utes like Rangers and Hiluxes often dominated the conversation but Collins said that’s not where the real risk lay.
“What the Government is really worried about is the heavy fleet,” he said.
"Our logistics, our farming equipment, all of that type of horticultural machinery, tractors, harvesters, the trucks that take the produce to the processor, from the processor to those distribution systems all around the country that ultimately then take all that through to our supermarkets.
"It's that part of the economy is very susceptible to prices in diesel."
A weaker New Zealand dollar added another layer of pressure.
Oil was priced in US dollars, and with the Kiwi dollar sitting much lower than during previous oil price shocks, importing fuel now cost more even before refining margins.
Diesel has historically been cheaper than petrol in New Zealand because petrol carried fuel excise duty while diesel vehicles paid Road User Charges (RUCs) instead.
Collins said the system was designed to balance costs between petrol and diesel users over time but that dynamic had become "distorted" by the recent spike.
"What we've got now is the Road User Charges staying the same and the diesel prices hiked up, so for those diesel operators, it's more expensive."
Collins emphasised that New Zealand was not facing a fuel shortage and the Government had fuel resilience plans in place.
However, the fact that the pressure was global meant there was little we could do.
New Zealand’s dependence on imported, fully refined fuel had also increased its exposure to global price swings.
Since the closure of the Marsden Point refinery, New Zealand no longer refined crude oil domestically, meaning international disruptions now flowed more directly through to local prices.
"We can monitor it, we just have to see how we respond to those things beyond our control, and control the things we can," he said.
"We're kind of at the mercy of these international markets now."
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