Fuel competition isn't working as well as it could

Commerce Commission analysis shows the lag in dropping petrol prices costs motorists 15-million dollars every year.

It's found retailers are quick to put prices up when their costs increase - but slow to reflect easing markets with cheaper fuel at the pump.

Commissioner Bryan Chapple says motorists can take things into their own hands and shop around, which will drive lower prices, already seen with un-staffed petrol sites.