Diesel prices have soared in recent days - and experts warn it could get worse.

The price of diesel has risen above 91 octane petrol for the second time in months. On Tuesday, the average price was at $2.65 a litre, while 91 was at $2.56.

According to fuel price tracking app Gaspy, diesel was as expensive as $2.88 in Invercargill, $2.75 in Christchurch, $2.68 in Wellington, $2.88 in Whangarei and a whopping $2.99 in Auckland.

AA principle policy adviser Terry Collins said the price is being heavily affected by a range of factors.

"Diesel is a middle distillate; it's similar to Jet A-1 kerosene and is very similar to heating oil that's used in the Northern winter hemisphere," Collins told Focus. "All those are in very much demand right now because, globally, the reserves and inventories of that stock have been very low.

Diesel prices have surpassed that of 91 octane petrol. Photo / Michael Cunningham

"It's been a whole combination of the Russian-Ukrainian war, tight supplies in Europe, our weakened dollar and high inflation."

"I do see it short-term, in the next week or so, getting slightly worse […] Coming up to December, we know that's there's sanctions applying to Russian oil on the water […] so it's not a rosy picture for the last quarter of this year and the first quarter of 2023."

Collins added there was very little that could actually be done to drive down the price.

"They've already reduced the fuel excise duty by 25 cents […] reduction in road user charges and are spending about a billion dollars topping up the national and transport fund to maintain and build our roads," he said.

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"And we all know what's been happening with potholes and the cry towards the Government to do more about that, so they can't win either way."

The Costco petrol station in Westgate. Photo / Michael Craig

When it came to where the best deal could be found, Costco in Auckland's Westgate won by a landslide, coming in at $2.42 a litre.

The US retailer opened its petrol station back in April and was an instant hit, with Kiwis lining up to grab a membership well before the warehouse itself had an official opening date.

But it too has gone up in price - Costco having previously offered diesel for $2.10 a litre.

"Costco's fuel pricing policy follows the same philosophy as our warehouse, to ensure we offer our members value on their fuel purchases," Costco's New Zealand managing director Patrick Noone told the Herald.

"To keep our prices low, we work closely with our fuel supply partners to have a very strong relationship that will benefit our members."

Meanwhile, there are a range of ways to save on fuel, whether driving on petrol or diesel.

The AA says to remove unnecessary weight from your vehicles like things in the boot or roof racks, to drive smoothly and to cut out short trips where you can.