Diametrically Opposed pricing policy

I live in New Zealand. My question is simple. Why does it cost significantly more (as much as double) to travel the same distance, just in oppsite directions? Case in point; Air New Zealand regularly have return flights from LHR to AKL for NZ$1500 or less. But to travel return from AKL to LHR, a price like this is unheard of. Can someone explain why Air NZ charges their NZ customers at least 50% more minimum. And I’m not aiming this squarely at Air NZ, other carriers are just as guilty.