The competition regulator has asked Australia's major petrol retailers to explain their high retail margins, after its latest report found motorists were not fully benefiting from crude oil price falls.

Key points: Petrol prices averaged 124.4 cents per litre in the five largest cities in the December quarter

Petrol prices averaged 124.4 cents per litre in the five largest cities in the December quarter Retail profit margins on petrol were the largest on record last quarter

Retail profit margins on petrol were the largest on record last quarter From May motorists will be able to access near-real-time petrol price data

The Australian Competition and Consumer Commission (ACCC) released its fifth quarterly report into the Australian petrol industry this morning.

The report found that while quarterly average prices were lower than in previous quarters, they were not as low as might have been expected given the slump in crude oil prices.

It found that the quarterly average price in the five largest cities (Sydney, Melbourne, Brisbane, Adelaide and Perth) was 124.4 cents per litre, which was 8.8 cents per litre lower than in the previous three months.

The ACCC noted that Brent crude prices in December 2015 were at their lowest level in more than 11 years, and were significantly below their long-term average.

ACCC chairman Rod Sims said high refiner margins during the quarter had limited the benefits for drivers.

"Motorists are not fully benefiting from low crude oil prices, as they use petrol in their vehicles, not crude oil, and the difference between crude oil prices and international refined prices (i.e. the refiner margin) in 2015 was high," he said.

Mr Sims said while elevated refiner margins were largely outside Australian control, the excessively high gross retail margins during the quarter were firmly in the ACCC's sights.

The ACCC's September quarter 2015 report noted that quarterly average petrol gross indicative retail differences (GIRDs) in the five largest cities stood at 11.8 cents per litre.

That was their highest level since the competition regulator began monitoring prices in 2002.

In the December 2015 quarter, the GIRDs had increased to 12.4 cents per litre.

"The ACCC believes that retail prices have been unreasonably high in the second half of 2015 and in early February 2016, I wrote to the major petrol retailers seeking an explanation for the high retail margins," Mr Sims said.

He said he was hoping to receive their responses soon.

Near-real-time price information soon to be available

Mr Sims said successful court action brought by the ACCC at the end of last year would see more transparency in relation to petrol prices in the coming months.

From May this year, price information will be made available to consumers, free of charge, on an almost-real-time basis, at the same time as it is received by petrol retailers.

In addition, the pricing information that petrol retailers exchange would also be made available to third parties, including app developers and motoring and consumer organisations.

"Making this pricing information constantly available to consumers will improve price transparency and allow them to make better informed purchasing decisions," Mr Sims said.

"That will therefore create greater competition in petrol pricing, and hopefully lead to downward pressure on retail margins."