AA petrol watch spokesman Mark Stockdale said prices dropped 4 cents a litre this week.
"It went down 2 cents a litre on Monday and again 2 cents today," he said.
The reason for the drop in prices was because of a "combination of a drop in commodity prices and the improving exchange rate", Mr Stockdale said.
The drop in the national petrol price comes after falling prices in world oil - Agence France-Presse (AFP) reported.
Oil prices in the US have fallen as "traders awaited the next day's US government report on commercial inventories, expecting an increase that would add to abundant global supplies", it was reported today.
Prices were under pressure as "Saudi Arabia's crude stockpiles were at record highs and expectations of increased Iranian output with the potential lifting of Western sanctions on the Islamic Republic", AFP also reported.