Why Five Ascot Servos Are 26 Cents Apart on Diesel While Four in Darkan Match to the Cent
Every year I put the same question to my Year 11 class: if two shops sell exactly the same product, why would anyone pay more at one than the other? Today's diesel boards out of Western Australia make a better worked example than anything in the textbook.
As of 2pm AEST on Monday 24th Aug 2026, which is midday in Perth, diesel across 1,215 WA stations averages 255.6 cents a litre. That is close to the pack, with Victoria at 253.2, NSW at 252.5 and Queensland at 254.3. With the states that bunched, the interesting numbers are inside WA.
Five servos scattered and four in step
In Ascot, the suburb between the Swan River and the airport approach, five servos are selling diesel at anything from 231.7 to 257.9 cents. That is a 26.2 cent spread within a few minutes' drive. Fill a 70 litre ute at the dearest one and you pay about $18 more than at the cheapest.
Now go roughly 200 kilometres south east into the Wheatbelt town of Darkan. Four servos there are posting diesel between 241.6 and 241.9 cents. The whole spread is 0.3 of a cent.
Same state, same fuel, same 24 hour price rule. So why are prices all over the shop in one place and within a fraction of a cent in the other?
The law of one price and where it breaks
Economists have a tidy idea called the law of one price. In a perfectly competitive market, identical goods sell for the same price, because anyone charging more loses every customer to the shop next door. If the law held, Ascot would look like Darkan. It fails for two reasons, and Ascot has both.
The first is search costs. Think of it this way: a bottle of water costs a dollar at the supermarket and four dollars at the airport, not because the water is different but because the airport customer has no time and no alternatives. Servos on the airport approach see a steady stream of hire cars that have to go back full, and those drivers do not shop around. The most likely explanation for a 257.9 cent board in Ascot is simply that enough drivers will pay it.
The other is who is doing the buying. A highway servo and a suburban corner servo can share a postcode, but the corner site lives off locals who fill up every week and remember what they paid last time. A local who feels overcharged drives past next week. A hire car on its way to the terminal never comes back.
Darkan has neither problem. In a town of a few hundred people, every driver knows every board, and every operator knows that being three cents dearer than the servo down the road means an empty forecourt by lunchtime. The key factor here is visibility. When everyone can see everyone, prices converge, and they converge fast.
Why WA's 24 hour rule makes the gap so clear
Under FuelWatch, servos have to lock in tomorrow's price by 2pm today, and that price then holds for 24 hours from 6am. So the Ascot spread is not noise. The servos at either end of that 26 cent gap chose their numbers yesterday afternoon knowing they would stand all day.
In the eastern states a servo can move its board three times before lunch. In WA, what you see at breakfast is what you pay at dinner, which is why the best time to fill up in Perth has less to do with the hour and more to do with which board you drive to.
Where Perth's cheaper diesel is today
The cheapest metro diesel today is in Kwinana Beach at 228.7 cents, though its six servos run up to 244.9, so the suburb name on its own guarantees nothing. Landsdale in the northern suburbs has a servo at 229.3 and a similarly wide 20.6 cent spread.
Beckenham behaves the way Darkan does: three servos, all at 231.3 cents, 24 cents under the state average. Bassendean starts at 231.3 as well, and Forrestfield sits in a tight band from 235.7 to 243.9. Further out, Wanneroo averages 240.2 across eight servos and Maddington 241.6 across nine. Outside the metro area, West Busselton starts at 232.5 and Pinjarra at 236.9.
What to do with a wide spread and a narrow one
In a suburb with a tight spread, like Beckenham or Darkan, stop looking and fill up. In a suburb with a wide spread, like Ascot, Landsdale or Kwinana Beach, the suburb average is close to useless and you need to look at the individual boards. Check today's diesel prices before you leave the house, and run the savings calculator if you want to see what a 26 cent gap adds up to over a year of fills.
The answer my class usually gets to is that people pay more when they cannot see the cheaper option, or cannot be bothered finding it. Ascot's dearest board is betting on both, and enough drivers keep proving it right.
