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.
A wiki page would be fine, feel free to make one.
I recall Mobil Trentham is often a few cents cheaper, but haven't been down there in a long time.
Gull seems to force the competitors to keep their prices down to compete, so in most locations where Gull is cheaper, the others will have to drop their prices by a bit to stay inline.