It doesn’t take long for a bout of ACCC toothpick-rattling to wear off at the petrol pump – we’ve just seen one of the weirder episodes of petrol pricing outside a holiday long weekend with refinery margins around $12 a barrel.

What’s more, the integrity of whatever moral suasion the ACCC might claim will be sorely tested this week. If the petrol price peak isn’t at least five cents below last Thursday’s effort ($1.33 in Sydney, $1.30 Melbourne, $1.24 Brisbane), the petrol companies will effectively be telling Graeme Samuel to go stick to rubber-stamping Murdoch newspaper acquisitions.

I’m normally rather sanguine about petrol price gyrations, but the pain in the tank over the weekend struck me as particularly odd given the strong Australian dollar and an oil price that had only just started to strengthen after going nowhere for weeks.

A chat with FUELtrac general manager Jeff Trotter and a quick tour of the Australian Institute of Petroleum and Shell websites confirmed that petrol prices should be falling rapidly. The A$ Singapore refinery price that Australian petrol pricing is based on peaked back on 9 March at nearly $100 – meaning the petrol company’s rolling seven-day average has been rolling remarkably slowly and with a fat refinery margin.

The Shell website is a remarkably telling service. If you scroll down to the bottom of the linked page and click on the average unleaded prices for the various capital cities some intriguing discrepancies appear. For instance, the peak of Sydney’s weekly price cycle jumped three cents last week while Brisbane and Melbourne were steady. And for a real laugh, have a look at the Darwin numbers – there’s no cycle there, only rising prices whatever the oil price and Australian dollar might be doing.

Memo ACCC: time to start finger wagging again.