Fortunately, there are some great apps out there. One of which is Gas Buddy, an app that has come to Australia from the US. After selecting your preferred fuel type, Gas Buddy will show you the price of fuel in your area and help you find the cheapest available. The difference in prices will vary from area to area and can range quite dramatically.
Prices are crowd sourced so it is constantly updated by its users. Gas Buddy offers points to fuel price reporters and these points can be used to enter a draw to win free fuel.
Gas Buddy is available for Apple and Android devices.