The international oil price plunged late last year, motorists enjoyed a brief relief on the cost of filling the tank - but not everywhere and not as fast as the wholesale price was falling.
Some prices remained significantly higher and some motorists waited significantly longer to see any benefit from the oil price drop and the Australian Competition and Consumer Commission wants to know why.
The ACCC announced an investigation as it released its first quarterly petrol report yesterday, revealing just how much motorists around the country have been paying for their fuel.
In January Australia's major capital cities enjoyed a reduced monthly average price for unleaded fuel not seen in five years.
The ACCC's investigation will aim to explain both the price differential and the delays in passing on the decreased international price
The price of unleaded fuel dropped a total of 42.9¢/L from June 2014 to January 2015 in most capitals.
But regional towns across the country continued to pay on average 12¢/L more. The report found the difference in unleaded fuel price between regional and capital cities was 5.4¢/L in June last year, and ballooned out to 17.5¢/L by December.
The ACCC's investigation will aim to explain both the price differential and the delays in passing on the decreased international price.
Motoring advocacy group RACQ is now calling for the ACCC to act on the findings from the report.
"The key question now is, what will the ACCC and the Federal Government do about it?" a spokeswoman said.
"This ACCC report highlights the problems RACQ has been campaigning against - regional motorists pay more than city drivers and Brisbane drivers paying more than those in the other state capitals."
Petrol prices in the state's southeast increased earlier this week after a small number of service stations upped their prices to match the now-rising oil price.The average price for unleaded fuel in Brisbane yesterday was at 123.6¢/L, however prices in Melbourne were still around 118.