A delay in dropping petrol prices is costing motorists $15 million a year at the pump.

The Commerce Commission's analysis of fuel monitoring data shows retailers are quick to put prices up in response to increased costs, but slow when it comes to bringing prices down when oil prices fall or the exchange rate changes.

"We can see clear evidence showing that fuel companies maintain temporarily higher margins after a decrease in their costs, lasting up to two weeks - at great expense to Kiwi motorists.