Diesel prices in Perth have dropped about five cents a litre since the start of the year, for instance, but that comes against a backdrop of much sharper falls worldwide.

It is clear that the recession in China and South East Asia generally, as well as around the world, is reducing global demand for diesel and this is causing world diesel prices to fall, he says.

Petrol Commissioner Joe Dimasi can only “watch” diesel prices because he lacks the power to call the oil companies in for an explanation as to why diesel prices have been slow in falling, Mr Zumbo said.

Mr Dimasi also lacks the power to subpoena oil company records to find out what "games" oil companies are playing on diesel prices.

"Once Mr Bowen gives Mr Dimasi these new powers on diesel, Mr Dimasi will be able to call in the oil companies and subpoena documents to get to the bottom of the oil companies’ failure to cut diesel prices as much as they should," Mr Zumbo said.

Mr Dimasi told WAtoday.com.au that there has been "phenomenal growth" in the use of diesel worldwide, including Australia.

International prices prior to last week had gone up more quickly than had those of unleaded petrol – an "international phenomenon" over which Australia has no control.

However, he believes diesel price falls have been in line with wholesale prices.

It is also difficult at the moment, to compare historical price information on diesel because new specifications for the fuel were introduced on January 1.

All diesel must now have a sulphur content of a maximum 10 parts per million, compared to 50 parts previously accepted.