These rules are based on a checked baggage concept,
which is from when a bag is checked in until it is picked up, as such there may be various carriers allowance and charges applied across a customer’s entire ticket url:
http://www.airnewzealand.co.nz/checked-in-baggage
Doesn't this effectively mean NZ only applies the MSC on journeys we're the passenger is directly connecting from another airline without collecting baggage? e.g. ZRH-LAX-AKL and not ZRH-LAX (stop) LAX-AKL?.