The stars have aligned to drive petrol costs down, says a fuel price tracker.

Crude oil prices have plummeted in the wake of US president Donald Trump's ever-changing tariff policies.

"They started to rise at the beginning of April but now we're seeing the beginnings of what looks like quite a sharp drop off," said Mike Newton, director of Gaspy, a website that compares prices between fuel stations.

"Of course a lot of the impacts of what's going on with the US is still to be seen, because a lot of it is speculative."

He said several factors had come together to create the downturn.

"They're talking about a downturn in demand for gas because of the economic uncertainty, and that I think is still to be realised," he said.

"At the same time you've got OPEC increasing supply, so that will bring prices down, and at the same time the US dollar is weakening so it's kind of like a triple whammy."

Newton hoped to see prices drop even further.

"Hopefully it means we see 91 heading down to $2.50 in the not too distant future, but it remains to be seen," he said.

"I mean, everything is so uncertain in the US and all the things bringing prices down at the moment could easily flip on their head and start to push prices back up because they change their mind every other day."

He expected the decline to continue for a while longer.

"I think prices are still going to continue to drop in the next few weeks, but in saying that I don't think you're going to see massive drops. I think it's going to slowly ease its way down."