One thing that could confuse the NZ car ownership data, around the time they introduced the helmet laws, is this:
Pre 1992-ish, NZ had very strict laws on importing vehicles. It meant that most of the cars on the road were either Aussie muscle cars or very old British cars from the 50s/60s/70s e.g Ford Cortinas, Vauxhall Chevettes, Triumph 2000s etc. Cars were scarce, and prices were high.
In the mid 90s, these import laws were relaxed and the market was flooded with cheap Japanese imports. So many of the old cars were scrapped and replaced by more reliable Japanese models.
NZ is (and has always been) very much a car based society. Cycling rates are only high in the city of Christchurch (flat/student population).