Costco dropped the price of unleaded petrol to 97.7 cents a litre on Tuesday afternoon, soon after dropping it to 99.7 cents, 35 cents cheaper than last week's average for Canberra.

CommSec chief economist Craig James said when comparing terminal gate wholesale prices with Costco's discount, it was likely it was sold at a loss.

Costco country manager Patrick Noone would not comment. "I can't comment on that. It is market-driven," he said.

A Woolworths spokesman said as a price follower, not a price setter, the supermarket chain would continue following Costco at Majura Park. "We try to match the market and they are the market at the minute," he said.

When Costco opened its doors to motorists on Wednesday morning, prices had risen beyond the dollar mark to 104.7 cents - which was up to 20 cents a litre cheaper than in other parts of Canberra.

Mr James said Commsec had a record of prices going back to 2004, which showed Canberra's petrol price in January 2005 was an average 95.2 cents a litre.

Canberra had not been able to sustain a discounting cycle, and had been more stable on price than other capital cities, he said. "It gets down to number of operators and competition factors."

"Costco's arrival throws down the gauntlet to established players. If you get a market which is a little bit sleepy, gets woken up by a new competitor, the game has changed. It's a case of you may have to look to new techniques to get people in the door.

"Over recent years the important point to a petrol retailer is to get the people through the doors, because they are not making too much at the pump. What they are trying to do is generate some sales in bread and milk and convenience-type items."

Marketing efforts at super-low levels was good for marketing, but in terms of sustainability, upstream influences came into play like the wholesale price, the Singapore gasoline price, Mr James said.

"They are continuing to fall. As yet the full benefits of the global decline have not passed through to the Australian market.

"It would take a brave retailer to put prices 10, 15, 20 cents higher than everyone else. Once prices do start stabilising on global markets, we may see the discounting cycle back."

Motor Traders Association ACT spokesman Peter Taylor, a former independent service station owner, has warned over many years Canberra would drop out of the petrol-discounting cycle because of a lack of independent operators.

In the past independent operators bought fuel off the wholesale market, but none existed in Canberra now, Mr Taylor said.

"I believe [Costco's discounts] are a good thing. It is the only factor in the marketplace that is going to pull the price down and affect the oil companies and grocers' monopoly that they have bought themselves into," he said.

"Costco is not going to change the entire Canberra market, because they have only one location. The nature of petrol is some people are concerned about price and they tend to be very vocal. Most people buy from their local [service station]. Convenience is important."

Traffic on Majura Road was congested on Wednesday night with many Canberrans travelling across town to make the most of savings at the bowser.

The ACT's planners had let Canberrans down because of too few service stations on the western side of the city, he said.

"You would not want to live in the Weston Creek area, or in any of the new suburbs and be looking for a service station, would you?"

Convenience was important for most people and Costco would have a moderating affect on the market, Mr Taylor said.