I find this quite perplexing. Just finished a holiday down south and when I left Auckland, 91 was around $3.01 a litre. At Pak'N'Save Lower Hutt I saw it for $2.76 and in Christchurch it was $2.81 (from memory).
Got back to Auckland and I think it was $3.02.
But why such a variation? Why is Whangarei so expensive? Is it just because there's little-to-no competition? Does Marsden Point being nearby have no bearing on transportation costs?