The big question to be asked is why didn't it have sufficient fuel to continue to the only other commercial airport in New Zealand suitable for the A340? I.e. Christchurch.
The big answer: Ohakea is approved as an alternate for CX ops into Auckland, although it is not commercially preferred. If the forecast for Auckland indicates a diversion is likely, then Christchurch would probably be planned as the alternate.
I don't know the details, but I suspect the forecast for this flight indicated reasonable conditions for the planned ETA. Consequently, Ohakea was planned as the alternate. The aircraft subsequently arrived at Auckland, was unable to land due to unforecast inclement conditions and then diverted to the planned alternate. Honestly, is that such a big deal??