Jacinda Ardern having trouble trusting fuel companies after leaked BP email

The Prime Minister says she's having trouble fully trusting fuel companies after a leaked email showing BP bumped up petrol prices to stem losses from nearby stations.

"When you see evidence like that it doesn't give you good cause to," Jacinda Ardern told The AM Show on Tuesday morning.

"Based on what I saw [on Monday] I think consumers are raising good questions. Whether or not I trust them across the board, that's a different issue.

"But on fuel pricing I think we need greater transparency because what's happening there just doesn't feel fair."

The BP internal email outlined plans to increase prices at the Paraparaumu, Kapiti and Levin stations, after reducing the price in Ōtaki.

"We have already increased all three sites mentioned by 5cpl [cents per litre] and have found that the Z [Energy station] in Paraparaumu has already matched our pricing," BP pricing manager Suzanne Lucas wrote in 2017.

"Over the next couple of weeks we will continue to try a number of tactics in the hope of reducing the pricing gap between Ōtaki and its surrounding regions."

Ms Ardern told host Duncan Garner the Government has "started off the process of trying to allow the Commerce Commission to be able to undertake market studies".

"At the moment, it's pretty hard to build an evidence base of course you'll have companies who will cry foul and say that they're doing absolutely nothing wrong, as BP has done.

"But consumers will say it just doesn't feel fair. And so we've got to give those powers to the Commerce Commission and that's why we're working on changing the law as we speak.

"On the face of it, they're not breaching any law but they're certainly breaching people's good faith. I've observed things across this industry for some time that have caused me to raise eyebrows."

BP has responded to the criticism from consumers saying petrol prices have always been "highly competitive".

"We adjust our prices in response to local competition, particularly in instances where discounting has become unsustainable, which is what occurred in the lower North Island last year," a BP spokesperson said.

"We believe competition on price will remain a key characteristic of the New Zealand fuel market in the coming years."