Caltex Australia hit back against Mr Sims' comments made earlier this week that "Canberra consumers were paying more than they should" for fuel.

Caltex spokesman Sam Collyer defended higher pricing in Canberra saying it was a small market and petrol retailers were bound by different pressures to counterparts in other city locations.

"For comparison, Sydney has more than 800 service stations servicing about 5 million people, compared with less than 60 service stations in the ACT servicing a population about 8 per cent of that size," he said.

"With significantly less customers, service stations and volumes of fuel sold, the gross margin on fuel must logically be higher for the local operator to make a return on investments and stay in business."

However, the head of the competition regulator has questioned the retailer's basis for inflated pricing.

Mr Sims said ACCC figures showed average annual gross retail margins, the difference between retail prices and published wholesale prices, were 10 cents per litre in most Australian cities and 16 cents per litre in the ACT.

"Larger capital cities, their margins are already high, but in addition to that you have an extra 6 cents in Canberra," he said.

"Transport costs can only be one or two cents of that."

Mr Sims said quick calculations showed the ratios were close, with Sydney having one petrol station to every 6667 customers and Canberra one to every 6250. Canberra is only four petrol stations short of the same ratio.

"On Caltex's own numbers there is not much difference in the number of service stations per head of population," he said.

"And putting a bit of quick logic around this, Canberra drivers drive further than Sydney drivers. I doubt the difference in fuel per service station, but I'm happy to see those numbers."

Analyst are predicting a petrol prices in response to the OPEC deal to cut output and the ACCC will be looking out for price gouging through the Christmas period.

However Mr Sims said all Australian motorists would be slugged with the flow on costs and he didn't expect ACT motorists would feel the pinch any more than elsewhere.