Drivers bear $5 bowser rip-off

Motorists are being ripped off to the tune of $5 a tank, delivering Big Oil a potential windfall of $30 million a week.

Analysis by News Corp Australia reveals the margin on average national petrol prices is the highest since at least early 2010 - and possibly much longer. This is because the retail price is not dropping at the same pace as the wholesale price.

News Corp Australia revealed on Saturday Melburnians had benefited more from lower world oil prices than motorists in other states.

Australian Institute of Petroleum (AIP) figures reveal the gap between retail and wholesale prices of unleaded has been as much as 19c per litre in the past month, compared with a long-run average of 8-10c per litre.

It means drivers of cars such as the Mazda6 have been forking over as much as $5.50 more a tank due to the extra margin.

The Australian Competition and Consumer Commission has estimated that every 1c per litre extra motorists pay fattens fuel companies' profits by $190 million a year - about $3.6 million a week.

This suggests that when the margin is 9c per litre more than the historical norm, the bottom-line benefit is $33 million a week.

AIP data also shows the raw price of petrol - known as MOPS95 - has plunged 15c per litre since the end of September to less than 64c per litre. Yet the national average retail price has fallen just 1c per litre in that time.

Were it not for a big drop in Melbourne, the national average would be up. In Victoria, the situation is not as bad - prices have fallen about 12c per litre. Melbourne has enjoyed a 14c per litre dive.

"Clearly the retailers are on notice to pass through the savings," said CommSec chief economist Craig James, who closely monitors petrol prices at a national level.

"If we don't get a fall ... there'll be questions asked." Another industry watcher, who asked not to be named, said: "Prices are not as low as they should be." The AIP data was published just before last week's 10 per cent dive in the price of Tapis oil. That slump could further expand the gap between wholesale and retail petrol prices.

The ACCC has begun Federal Court action against most petrol retailers, alleging price co-ordination.