Market price for fuel and exchange rate.
A day after the nation's petrol watchdog said Perth petrol prices were behaving "strangely", BP has hiked them to their highest level in two months.
The oil major, which controls the Kwinana refinery, is putting up unleaded petrol prices at several of its service stations today by 18 cents a litre.
At 119.9 cents a litre, they will be 15.1 cents above the metropolitan average, itself rising again despite falling prices internationally.
The RAC has called the price hike disgraceful.
It is the third time in a month that BP has hiked its prices against the trend. In mid-December it raised them by 12 cents a litre, ending a two-month fall in the city.
That sparked higher average prices for Perth motorists for several days as other retailers followed suit. They did not fall again until just before Christmas.
BP again hiked prices by 12 cents a litre on January 7, an action which was followed by Caltex the next day.
The Singapore benchmark price, used to set Australian prices, has risen in the past week by about five cents a litre but there is usually a lag of between one and two weeks before prices are passed on.
It was at yearly lows of 36 cents a litre about three weeks ago.
The Singapore price's rise is at odds with the world crude oil price.
Competition expert Frank Zumbo, an associate professor at the University of NSW who predicted both previous price rises on WAtoday.com.au, said the Singapore price "appeared to be defying gravity".
"The so-called time lag (between changes to the Singapore and Australian prices) mysteriously disappears when the Singapore benchmark price goes up and that magically reappears when the Singapore price is on the way down," he said.
"A sharp rise (in the Singapore price) will also conveniently help oil companies boost their local profit margins at motorists' expense."
With the world oil price having fallen to $36 a barrel and a stable Australian dollar, Perth motorists should be seeing falls in retail prices, Mr Zumbo said.
But Perth motorists have had to contend with rising unleaded prices on 12 days in the past month. BP's price of 119.9 cents a litre has not been the average in Perth since November.
RAC head of member advocacy Matt Brown said motorists needed to use their "collective consumer power" to punish BP for the hike "and send a message to petrol retailers that they won't be gouged on petrol prices".
"If you see a BP station... charging 119.9 cents a litre simply drive past it," he said.
There was no justification for Perth motorists to be hit with radical jumps in the price of petrol.
Petrol Commissioner Joe Dimasi told WAtoday.com.au that while petrol prices in Perth were behaving "strangely", mainly due to the price cycle disappearing, he had no concerns yet.
However, he was "keeping a close eye" on what went on in the Perth market, given that the Singapore and world prices continued to head downwards.
"If we don't have an explanation, we'd have to start looking at the behaviour of the participants, but the indication to me at the moment is that whoever is trying to put prices up hasn't succeeded."
He was satisfied there was enough competition to stop BP hiking wholesale prices beyond a reasonable level, despite it owning the Kwinana refinery, with the likes of WA-owned Gull able to import fuel and others able to do deals to get lower prices.
FuelWatch says 195 service stations will be selling petrol below the average price.
