Monday has emerged as the cheapest day to buy fuel in Perth, authorities say, bringing an abrupt end to a long-running cycle.

Commerce Minister Michael Mischin said Monday had replaced Wednesday as the cheapest day to buy fuel, based on an analysis of price trends by Fuel Watch in recent weeks.

Mr Mischin said the situation changed around the end of May when Caltex stations suddenly broke the previous price pattern, which had existed for about five years.

"There was a bit of a settling in period but it seems the trend now is for Mondays to be the cheapest, Tuesdays to be the most expensive and the price dropping over the course of the week from Tuesdays," he said.

Royal Automobile Club WA corporate affairs manager Will Golsby said the organisation's members had expressed concern over the change.

"I think members are worried about what the cycle means and that is where we need some certainty on the information provided," he said.

"We don't have a full understanding of why the cycle has changed ... and we want to understand why this is happening.

"It has changed after a period of certainty and we don't know what the future looks like."

Mr Mischin said the change in pricing structure may be a "ploy to catch consumers on the hop", but denied there was any evidence of anti-competitive behaviour.

"I think the [Australian Competition and Consumer Commission] looks at this sort of stuff anyway ... but I have no evidence of that," he said.

"Of course they'd be looking at ways to try to maximise the amount of profit that they make, whether that means they are being anti-competitive or in collusion is another matter."

Mr Mischin also released figures for the cheapest unleaded fuel sites for the first six months of the year.

Eight of the 10 cheapest sites were north of the river, with three in North Perth and another in nearby East Perth.

The cheapest average unleaded price from January to June was 120.5 cents per litre at a Coles Express site in North Perth.