Petrol prices are becoming increasingly more difficult to predict, but the RAA says that's because the discount fuel cycle has changed.

What was once a seven day cycle has now expanded to ten days, and RAA fuel expert, Chris West, says it's cheaper to fill up between days eight and ten.

"If they see a spike in the price appear, you have about eight hours to react to that and try to fill up, by hunting around and find that good price that can be 20 cents lower than the spiked price," he told 7News.

• For more information on the best time to fill up, head to the RAA fuel price page here.