Perth's petrol price cycle has been overturned after a fuel retailer shifted the timing of its weekly hike, and others followed suit.

For the past four-and-a-half years, the cheapest day for Perth motorists to fill up was Wednesday but that suddenly changed three weeks ago when the city's biggest retailer, Caltex, upped its prices on a Tuesday.

FuelWatch spokesman Kyle Hyunh said it was unclear if a new price cycle was being established but in the meantime, consumers were paying the price.

"What we've seen this week is Caltex/Woolworths and BP, two other major brands, following Caltex's lead by hiking their prices on Tuesday," he said.

"This is the first major change in the cycle that we have seen.

"It's still unclear whether this will become the new price cycle, the norm.

"If they're able to confuse consumers, so consumers are not able to purchase on the cheapest day, then they will certainly benefit from higher volumes and higher profits."

FuelWatch said motorists should consult its website to find the cheapest fuel in their area.

"Fuelwatch is recommending motorists shop around," Mr Hyunh said.

As motorists grapple with unpredictable prices they are also paying much more for fuel.

A CommSec report showed national petrol prices were about 13 per cent higher so far this quarter, reaching 142.1 cents per litre last week.

That was likely to be the biggest quarterly jump in almost 25 years.

But economists said with oil prices stabilising, petrol prices were unlikely to rise much further.

"If the Australian dollar doesn't fall much, probably there will only be slight increases in petrol prices over the next few weeks and months," Bankwest chief economist Alan Langford said.

"There's not a lot of upward pressure on oil prices at the moment."