The competition watchdog wants Australia's major petrol retailers to explain their massive retail margins.

The Australian Competition and Consumer Commission (ACCC) has released a report showing motorists are not fully benefiting from crude oil price falls.

The report said quarterly average prices were lower than previous quarters, but given the slump in crude oil prices, they should be much lower.

The quarterly average price in Sydney, Melbourne, Brisbane, Adelaide and Perth is 124.4 cents per litre, 8.8 cents per litre lower than the previous three months.

But the ACCC says this does not match up with Brent crude prices from December 2015, which showed oil was at its cheapest in over 11 years.

“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,” ACCC chairman Rod Sims said.

He acknowledged that higher refiner margins were largely outside Australian control, but the excessively high gross retail margins are firmly within the ACCC’s sights.

It comes after the ACCC’s September quarter 2015 report, which noted that the difference between crude oil and refined petrol prices were at their highest level since the competition regulator began monitoring prices in 2002.

“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.

The ACCC is expecting to receive their responses soon.

The watchdog has also reported on moves to give customers more transparency on petrol pricing.

From May this year, near-real-time price information will be made available to consumers for free.

Additionally, pricing information normally exchanged between petrol retailers will also be made available to third parties, such as app developers and motoring and consumer organisations.