Just out of interest whats the strongest rivalry in New Zealand?
auckland v Christchurch ?
auckland v Wellington?
Auckland v Dunedin?
Across all sport, Auckland v Christchurch (Canterbury province).
Historically there's plenty of contrasts in the rivalry - metropolitan vs more rural-slanted, multicultural vs predominantly Caucasian, blue vs red, North Island vs South Island.. and I'm convinced that feeds into the rivalry being that much more intense than any other in NZ sport.
On just about every aspect, Auckland & Christchurch find themselves as opposites, which just feeds into a clash of cultures as much as sports.