All the major petrol companies cut their prices by 5c a litre yesterday. Shell was first, at 3.30pm, taking the price of its unleaded petrol to 176.9c a litre.
The rest followed soon after, with Gull bringing its price down to 173.9c.
Shell spokesperson Kylie Reeves said: "The price of crude oil has been steadily falling and this is continuing to flow through into the cost of refined product."
The last petrol price decrease was 8c a litre on Friday. Petrol has dropped more than 40c since July 17.
Mark Stockdale, of the Automobile Association, said: "The petrol cut is excellent and reflects continuing reductions in the product price, coupled with a steadying exchange rate - and we hope another drop will also be forthcoming for diesel."