The AA suggests there might be an ulterior motive for price cuts.

Motorists rejoice: Fuel prices have dropped 9c over the last week.

AA monitoring shows the latest 3c drop has taken the price of petrol and diesel 14c lower than it was a fortnight ago.

Spokesman Mark Stockdale said there was no clear reason to explain it.

He said some of the reduction was due to an increase in the value of the New Zealand dollar against the US.

READ MORE: Energy Minister Judith Collins vows to 'get to the bottom' of rising petrol margins

There had also been a fall in commodity prices - oil fell US$2 a barrel overnight.

But he said, even with those two factors combined. the "landed cost" of fuel to the petrol companies had not fallen enough to justify the reduction in retail prices.

"It doesn't add up to that 14c reduction at the pump. I'm not sure why petrol companies have dropped their prices as much as they have. We think they are basically cutting their margins. They are lower than we have seen for the past year."

He said it was possible that the companies were trying to get ahead of a government petrol price investigation, which is due to report back soon.

When Energy Minister Judith Collins announced the study in February, she said fuel margins has doubled over the past five years.

The Fuel Market Financial Performance Study was expected to be finished by the end of June.

Stockdale said petrol companies might hope to be able to claim their margins were not as high as the study suggested. "Everyone will see through this. Margins have been quite high for the last year. Dropping margins a couple of weeks before the announcement doesn't make any difference."

Z spokesman Jonathan Hill rejected that suggestion. "Z has been leading price cuts recently based on exactly the same factors that we always consider when setting prices – the global price of crude oil and refined fuel and the exchange rate," he said.

"We're looking forward to the report, whenever it arrives, and we have participated fully and robustly in it."