The Commerce Commission says the delay in companies dropping the cost of petrol when company costs fall is costing Kiwi motorists $15 million a year.

It's warning fuel companies it'll be watching prices when Auckland's regional fuel tax kicks in next month.

AA principal policy advisor Terry Collins says petrol companies will be acting quickly come July to avoid further criticism.

"I've been talking to contacts within the industry, and they're very keen to ensure that the Auckland motorists get their full 11.5 cents - one of the large majors has got a big team who are trying to get it done by midnight."

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