It was "tight-arse" Tuesday. Now it's frugal Friday, or sale-day Saturday - and no-one appears to know why.

It is the day of the week in Sydney when the petrol stations move like a herd and shunt their prices up more than 15 cents a litre.

The NRMA's monitoring of about 400 petrol stations showed Sydneysiders paid between 133.9 and 149.9 cents per litre this week, with the highest prices generally on the lower north shore and cheapest fuel found in the west.

NRMA spokesman Peter Khoury said trends in the pricing cycle over the last few weeks showed Fridays and Saturday mornings have replaced Tuesdays as the cheapest time to buy fuel, but it's impossible to explain why.

"The weekly price cycle is something that no one has explained to the public," Mr Khoury said.

"We are the only country in the world that has it and there is still no true definition as to why it occurs, so we would love to know as well.

"There was talk that it had to do with the marketing strategies of the oil companies, the fact that when one moved they all move, so there's a whole raft of different reasons that have all been floated around.

"But as far as we know, there is no formal reason as to why we have a weekly price cycle."

Service Station Association chief executive Ron Bowden said unrest in Libya created a nervous market and drove prices up by about eight cents a litre.

He said motorists could expect even higher prices.

"We are in for a period of elevated prices and if you can tell me how long the Libyan crisis can go on for, I can tell you how long we're going to have elevated prices," Mr Bowden said.

"That's the difficult thing, nobody knows.

"It's impossible to predict how long, it's impossible to predict how high."

But, according to the most recent figures on the Australian Institute of Petroleum website, Australia has some of the cheapest fuel in the world, along with North America.