What’s up with fuel prices

Aside from them being ridiculously expensive, I’d like to know why fuel prices vary so wildly between different regions and towns. I don’t think it’s “competition” because prices will usually be pretty set within a ballpark range within a geographic location.

In National Park today 91 is 301 per liter. An hour up the road near Taupo you can get it for 244 ~ 260. In Masterton it will be 260 ~ 280. Make it make sense.

Also, how come the big retailers can get away with increasing the price by the amount of a discount coupon so that when you use a coupon you’re paying what it should be. (I.e. NDP in Taupo charging 254 and Z charging 260 so it’s 254 after a new world 6c off coupon)