Single image at a petrol station sums up the fuel crisis gripping Australia - as a fresh warning is issued for millions driving over the Easter long weekend

Motorists forked out almost $4 a litre for diesel at a Perth petrol station after a simple but costly typo.

Independent retailer Burk tries to sell to cheapest fuel in the state to provide relief for motorists.

But its Carrington service station had the most expensive diesel at 397.3 cents a litre after a staff member accidentally entered the wrong price.

Petrol companies must report prices to FuelWatch by 2pm every day. Those prices are then locked in for 24 hours.

'We won't sell it at that, it's just a rip-off,' Burk managing director Umar Farooq told Nine News.

'We tried to rectify it with FuelWatch, multiple calls, multiple emails but we were just told, 'You can't sell it at the price other than what's been reported.'

Burk got around the costly mistake by offering $1.04 per litre off the advertised diesel price for a smile.

Families hoping to get away over Easter are unlikely to get cheaper petrol before the long weekend, while regional areas could be waiting weeks for relief.

The federal government has cut wholesale fuel prices by 26 cents a litre in a bid to head off the worst economic effects of the Middle East war.

But the change would not be felt straight away because service stations needed to sell all their older, higher-taxed stock before bringing in the cheaper fuel, according to NRMA spokesman Peter Khoury.

That process would likely take anything from a day or two for high-turnover metro stations to two or more weeks for some regional sites, he said.

'Once they buy new fuel, they will pass the discount on at that point,'Mr Khoury told AAP

The consumer watchdog would be watching closely to ensure service stations passed on the price cuts to consumers, Treasurer Jim Chalmers said.

But he reiterated the change would not kick in straight away.

'I want to manage expectations on that front because people shouldn't rock up at five past midnight ... and expect to see the full benefit passed on," he told reporters on Tuesday.

Former Australian Competition and Consumer Commission boss Allan Fels said while there was no law against price-gouging, public shaming would be a powerful tool to force fuel companies to do the right thing.

'The ACCC has no direct powers either to set maximum prices or to fine companies for excessive pricing or price-gouging," he told AAP.

'But the ACCC can publicly criticise someone that's not passing on the benefit.'

The watchdog would also need to keep a close eye on the "rocket and feather" effect on fuel prices, Professor Fels said.

'When costs go up, prices go up like a rocket. When costs go down, prices fall slowly like a feather to the ground,' he said.