Unleaded petrol has reached its highest price in almost four years across Australia's capital cities, according to the Australian Competition and Consumer Commission.

The price of unleaded is almost 12 per cent higher than this time last year at an average $1.45 per litre across Sydney, Melbourne, Brisbane, Adelaide and Perth.

A jump in international crude oil and refined petrol prices are to blame for the price increase, the ACCC said in its report released on Tuesday.

The watchdog also pointed the finger at the Organisation of Petroleum Exporting Countries, or the OPEC cartel, for bumping up prices.

The ripples of the cartel's 2016 agreement to cut crude production is now being felt across the world - compounded by concerns of risks to international oil supplies.

The supply concerns are born out of renewed US sanctions against Iran, fears of spreading conflict in the Middle East and a political crisis in Venezuela, according to the ACCC.

A weaker Australian dollar was also contributing to the price hike.

"The lower AUD-USD exchange rate in the June quarter 2018 compounded the influence of the increase in international refined petrol prices on Australian retail petrol prices," the report said.

Brisbane was identified as having the highest retail petrol price averaging $1.48 per litre.

PETROL PRICE RISE BY THE NUMBERS:

* 2016/17 average: $1.35 per litre

* 2017/18 average: $1.45 per litre

* Average retail margin: 12.8 cents per litre (0.4 increase since previous quarter)

* Highest retail margin: Brisbane (15.5 cents per litre)

* Lowest retail margin: Adelaide (10 cents per litre)

* Fuel price apps and websites are said to be the best way for motorists to save cash

* Difference in country versus city prices: 4.9 cents per litre

Source: Australian Competition and Consumer Commission

Australian Associated Press