Australian Convenience and Petrol Marketing Association chief executive Nic Moulis said the two grocery groups had much greater market penetration in Canberra than elsewhere.

''There are 64 retail fuel sites in the ACT,'' he said. ''Coles and Woolworths have 31 of these, a total of about 48 per cent.

''Nationally they control 25 per cent of the petrol station sites and account for 50 per cent of retail petrol volumes.''

Mr Moulis said if the Canberra supermarket service station sites had that level of penetration they would own the market. There would be nothing left for the independents. While that was not the case, he said many independents were in survival mode and faced a future that was getting bleaker by the day.

Professor Phil Lewis, a professor of economics at the University of Canberra with a keen interest in competition and sales across the territory, agreed the big two dominated the local fuel and grocery market.

He said while the advent of Aldi and Costco had resulted in some inroads being made into their dominance of the grocery market, there was no sign of this happening in the retail fuel sector.

''It is fairly easy to convert space in a shopping centre to create a new [competing] supermarket,'' he said.

''The only way you can introduce competition into the fuel sector is by opening up new sites [for independents] on busy thoroughfares such as Belconnen Way, Ginninderra Drive or Hindmarsh Drive.''

Independent fuel and grocery retailer concerns that they were being squeezed out of the market was one reason the Australian Competition and Consumer Commission accepted court enforceable undertakings from Coles and Woolworths not to discount below 4¢ a litre last December.

The regulator has taken court action against Coles and Woolworths this week for allegedly breaching those undertakings.

Professor Lewis and Mr Moulis said historical factors and planning decisions had made the ''shopper docket'' marketing strategy used by Coles and Woolworths to drive petrol sales more effective than in other states. Service station sites had been pushed into the community hubs, often in close proximity to the supermarkets.

A Canberra motorist cannot drive 500 metres down a stretch of highway and have a choice of three or four service stations as is often the case in Melbourne or Sydney.

They believe that despite the upfront offer of a discount, the emergence of the duopoly with its stranglehold on the ACT fuel market was already costing Canberrans extra money.

''When you have a monopoly [or duopoly in this case] retailers can split the market up according to the ability (of particular areas) to pay,'' Professor Lewis said.

''Sydney is a highly competitive market and people tend to have lower incomes; petrol there is cheaper. In Canberra people use their cars a lot and incomes are higher.''

He agreed it was an example of the old adage that businesses would ''charge what the market can bear''.

Mr Moulis said Canberra had more supermarket petrol stations than elsewhere and that petrol prices here were generally higher than in Sydney and other parts of NSW.

''Other people have made the connection between those two factors,'' he said.