The price of petrol in some parts of the ACT plunged by 20 cents in a matter of minutes this morning, after a new operator opened its doors.

The fall was sparked by the opening of Costco's first petrol station in Canberra, which was selling unleaded fuel for $1.29 a litre to members.

Within minutes the nearby Woolworths petrol station had matched that price for customers with a four cents a litre discount voucher.

Last week's average petrol price in Canberra was $1.48 a litre, according to the Australian Institute of Petroleum.

It was the lowest ACT price in more than 40 weeks, but it was still 16 cents more expensive than the average price in New South Wales.

Alan Evans from the NRMA said the lowest prices could still be to come.

"The reason Canberra has high prices is because there's no real competition, now we've got some competition," he said.

"Amazingly we see Costco dropping their price, Woolworths dropping their price, I think the competition is really going to change the market in Canberra.

"I'm dancing on the roof because at last we're getting petrol prices somewhat near to Sydney and Melbourne prices."

Mr Evans said he was hopeful the significant fall in prices would be permanent.

"I hope it's not a first day stunt, I hope they keep the prices down and I hope the competition increases", Mr Evans said.

A spokesperson for Woolworths said it was unlikely the price would be matched at many of its other stations around the ACT.

"Woolworths has a policy of following the local price rather than leading it," the spokesperson said.

"We follow the rest of the local market as it moves, both up and down."

The NRMA said there was the potential for a even further drop in prices, provided the global price of oil falls and the Australian dollar remains steady.