The Australian Competition & Consumer Commission blames discounts and rising wholesale prices for a recent rise in petrol prices.
The ABC reports retail prices have jumped 25 cents per litre at some stations in the past few days.
ACCC petrol commissioner Joe Dimasi said higher wholesale prices and a fortnightly cycle of discounting had fed into the increase.
Mr Dimasi said the ACCC can explain, but not control petrol prices.
"The price is set by the market," he said. "The ACCC doesn't have a role in setting or regulating the price of petrol.''