Which petrol station in your area is consistently cheaper than the rest? Perhaps we could start a Wiki on this.
In my area (Central Auckland) it's BP Mt Roskill @ Carr Road, it's consistently 3c-5c cheaper than the rest and there's always a big queue of cars at rush hour. And it accepts AA Smartfuel.
I also noticed that Gull Atiamuri (On State Highway 1 between Tokoroa and Taupo) is about 10c cheaper. It was the cheapest by far while I was driving from Auckland to Palmy. However it's an unmanned station in the middle of nowhere so be wary of card skimmers.
And then of course there are the stations around Otara and East Tamaki Road, South Auckland.