Pump prices in the capital have been steady at 157.8¢ per litre and then 157.9¢ since mid-July, despite fluctuations in both the metropolitan and regional average prices, data from the Australian Institute of Petroleum shows.

In the same nine weeks, the national average price has bounced between lows of 149¢ per litre and a high of 158¢. The regional price has fluctuated from a low of 154.3¢ to a high of 156.9¢.

Price data for the year shows Canberra has had regular periods of steadily high petrol prices, largely above the regional average, and usually the third-most expensive capital behind Darwin and Hobart.

University of Canberra economics professor Phil Lewis said a combination of Canberra's perceived affluence and a lack of competition meant oil companies were free to set a higher price than in other cities.

''The way monopolies behave is they charge the highest prices in those markets where people can afford it,'' he said.

''So Canberra has a much higher income per head than, say, Sydney or Melbourne, and so therefore people are willing to pay the higher prices.

''With petrol, you have to buy from where you are. So you can actually have cheaper prices in Sydney, higher prices in Canberra, and the thing is, you can't move from one market to another.''

Based on 2012 Australian Bureau of Statistics figures for average car use in Canberra and average fuel consumption, the average ACT motorist has paid about $2126 for petrol so far in 2013, at an average price of about 152.6¢ per litre.

The same amount of unleaded fuel would have cost $80 less at the national average price of 146.8¢ per litre, and more than $120 cheaper at average Sydney prices of 143.9¢.

The disparity has been creeping up since 2010, when petrol prices in the capital ended the year almost on par with the national average.

In 2011, Canberrans paid an annual fuel bill about $20 more than the national average, and about $80 more in 2012.

At the current average prices for 2013, Canberra motorists are likely to have an annual fuel spend of $2990 - $110 more than an equivalent amount of fuel at the national average price, $170 more than Sydneysiders, and about $185 more than motorists in Adelaide.

Motorists in Darwin would pay about $3100 during the year for the same amount of petrol.

In the week leading to last Sunday, the average retail price of unleaded fuel in Canberra was 157.9¢ per litre, compared with 156.8¢ in Goulburn, 156.4¢ in Queanbeyan, 161.3¢ in Yass, and 154.4¢ in Sydney.

The last time Canberra had a run of petrol this expensive was through the middle of 2008, when crude oil prices peaked globally, and the pump prices in the ACT stayed above 158¢ per litre for nine weeks in June and July, hitting a high of 168.6¢.

Analysis from economists at Commonwealth Securities predicted the price of fuel nationally would likely fall about 3¢ per litre over the coming week.

Professor Lewis said the growing dominance of supermarket chains and discount deals in metropolitan areas allowed the big brands more control over prices and forced competition out of the market.

This had contributed to increased cost-of-living pressures for lower-income earners in Canberra, Professor Lewis said.

According to the Environment and Sustainability Directorate, there are 55 service stations in the ACT, most under the banners of either Shell, Caltex, BP or 7-Eleven.

On Thursday, Greens MLA Shane Rattenbury will introduce legislation to the Legislative Assembly calling for service stations to display actual fuel prices, not discounted prices, on their sign boards.

The government has indicated it is unlikely to support the bill.