You get used to it.
Drivers new to NZ naturally take a little time to get used to the differences as well as getting used to an unfamiliar city or area, but as you say you have driven in Europe and the USA we must really respect your opinion that the driving is bad here.
Not too keen on the roads in Auckland myself, but prefer it to clenching my teeth on the M25 or London's Circular roads.
Don't forget that you probaby hear more about the road accidents/fatalities here than in the UK because there is so little newsworthy items to report in the media. Still, must have been a bad day for you to see three pile-ups in such a short space of time.