A fuel boss says he expects prices to climb 15 to 20 cents a litre in coming weeks, after new developments in the United States - Iran war.

The conflict began when the US and Israel attacked Iran on February 28 this year and Iran responded with its own strikes on Israel and Gulf states that host ⁠US bases.

On Thursday, a tanker was reportedly struck by an unknown projectile off Saudi Arabia's coast.

It follows US President Donald Trump’s threat to bomb Iranian infrastructure in response to any attacks on ships travelling through the Strait of Hormuz -a crucial waterway for trade.

Waitomo Group chief executive Simon Parham told RNZ’s Checkpoint programme on Thursday he expected that those developments, as well as Ukranian drone strikes on Russian refineries, would push prices up.

"I'd forecast that we'd be going close to $3, just a little bit under. But on diesel, about $2.50. So that's where I'd sort of see it pan out in the next one to two weeks."

But Parnham did not expect prices to get as high as previous peaks in March and April.

"The reason for that is that there has been a rebalancing in the system, with crude coming from countries such as the United States, Brazil and Guyana across to refineries."

Parnham said China had also relaxed restrictions on petrol and diesel exports and were selling more to international markets, and markets no longer seemed to be reacting to every word Trump said. 

"I think the market... started to shrug their shoulders and say this is the new normal... the days of him talking the market up or down seem to have gone."

People in New Zealand had started to buy more petrol after demand plummeted in April and May, Parnham said.

"As we've come back into June, people started to purchase a little bit more but... when I look at our numbers people are topping up, they're not filling up.”