Petrol has dropped by five cents a litre and diesel by three cents a litre at Shell, Caltex, BP, Mobil and Gull service stations but the Automobile Association says up to another four cents could be taken off the price of diesel.

AA policy analyst Mark Stockdale said while the drop in petrol prices is "on the money", there is still a buffer in the diesel price.

"There may still be an opportunity for another cut of up to four cents a litre," Mr Stockdale said.

Earlier this morning and before the petrol companies made the decision to drop, Mr Stockdale said petrol companies could afford the five cent drop in petrol. He said up to seven cents could come off the price of diesel.

Mr Stockdale said there had been a drop in the cost of refined fuels on the international market and the exchange rate had been stable.

Mr Stockdale said the savings could be passed on to motorists.

"The oil companies have been saving on the imported price of fuels," he said.

Shell spokeswoman Kylie Reeves said the company reviews its prices daily and passes on savings "as quickly as we can".

Ms Reeves said the methods Shell uses to price their fuel "may be different" to the AA.

Shell, Caltex, Mobil and BP are selling 91 octane petrol for 158.9 cents a litre and diesel for 104.9 cents a litre.

Gull is selling 91 octane petrol and diesel for one cent less than the three larger companies.

All five companies say the drop in the prices at the pump have come on the back of a drop in the international market price of fuels.

Gull retail manager Graham Stirk said the international price has not dropped enough in diesel to justify a further four cent drop in prices at the pump.

BP spokeswoman Diana Stretch said she did not want to comment on how BP's price modelling could be different from the modelling done by the AA.

"It just becomes a he says, she says thing and we've been there before," Ms Stretch said.

She said the AA cannot know all of BP's costs.

"They don't know our business," Ms Stretch said.

Caltex spokeswoman Sharon Buckland said Caltex is a private company and the AA is an "external observer".

Mobil spokesman Alan Bailey said if there was room in the international price and competitors lowered diesel by four cents then Mobil would follow.

Shell
91 petrol: 158.9 cents a litre
Diesel: 104.9 cents a litre

Gull
91 petrol: 157.9 cents a litre
Diesel: 103.9 cents a litre

Caltex
91 petrol: 158.9 cents a litre
Diesel: 104.9 cents a litre

BP
91 petrol: 158.9 cents a litre
Diesel: 104.9 cents a litre