Average petrol prices increased by 12.6 cents per litre to 135.1 cents per litre in the December 2017 quarter, according to the ACCC's quarterly petrol report released on Friday.

The increase has pushed prices to their highest levels since 2015 in Sydney, Melbourne, Brisbane, Adelaide and Perth.

Meanwhile, gross retail margins in the cities were 14.2 cents per litre, an increase of 3.9 cents per litre from the previous quarter. That's the highest they have ever been since records began in 2002.

ACCC Chairman Rod Sims says shopping around and timing petrol top-ups during the price cycle encourages retailers to become more competitive.

"Motorists can fight back against these high prices by using fuel price websites and apps to shop around," Mr Sims said.

Prices in larger capital cities can vary by around 25 to 30 cents per litre close to the time price cycles increase, he said.

On petrol retailers' ever-increasing margins, Mr Sims said he accepts some of the increase has been due to increased costs, but said it doesn't explain the bulk of the increase.

Two thirds of the petrol price increase was caused by higher international crude oil and refined petrol prices and a lower AUD-USD exchange rate.

The ACCC report said the rest was due to high gross retail margins, which hit 8 cents per litre in the 2017 calendar year, costing Australian motorists an extra $750 million.

Australian Associated Press