Petrol prices should be four cents cheaper at the pump, according to the Automobile Association.

While the imported cost of diesel and petrol fell three to four cents per litre since the last price rise on March 14, prices remained unchanged during April.

AA records show diesel on $1.57 a litre and 91 octane petrol at $2.20 a litre in main centres.

"With a stable exchange rate, imported costs are down and motorists should have seen a drop in the pump price by now," said AA PetrolWatch spokesperson Mark Stockdale.

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He said monitoring of fuel company importer margins showed they were above the level where a drop in retail prices could normally be seen.

"We calculate prices should be about 4 cents per litre lower at the pump," Mr Stockdale said.

- HERALD ONLINE