The Automobile Association is accusing oil companies of widening the price gap between their 91 and 95 octane fuels by stealth.
For many years the gap was five cents, but recently, in most areas, it has risen to seven or eight cents. The difference in some cases is as high as 14 cents.
The AA's Petrolwatch spokesperson, Mark Stockdale, says members have been getting in touch to ask why they're paying more than they're used to.
He most motorists are not aware of the increasing gap because oil companies don't usually advertise the price of 95 octane fuel at petrol stations.
Both Mobil and Shell deny the move has happened by stealth and say the price difference has grown as a result of 95 octane being more expensive for them to buy.
Shell spokesperson Kylie Reeves says prices are monitored on a daily basis, and if the cost on the international market of refined higher octane fuel reduces, the company will bring down the price at the pump.